This is a savory tart with a flaky plant-based pastry crust, filled with slow-cooked caramelized onions and cremini mushrooms, bound with vegan cream and seasoned with thyme, garlic, balsamic vinegar, and sea salt. It takes about an hour and forty-five minutes start to finish, most of which is hands-off. Make it when you want something that looks impressive but doesn’t require unusual ingredients or advanced skills.
The short version of why this works
Two things matter here. First, the caramelization: the onions need a full 35–40 minutes over medium-low heat to go from sharp and raw to deeply sweet and jammy. Rushing this step with higher heat gives you soft onions with none of the flavor payoff. Second, the crust needs to be cold at every stage — cold vegan butter, ice water, cold hands if you can manage it. Warm fat melts into the flour instead of staying in distinct pieces, and those pieces are what create flakiness. Get both of those right and the rest of the recipe is straightforward assembly.
Common problems and fixes
- Soggy bottom crust: Blind-bake the crust before adding the filling. Line it with parchment, fill with dried beans or pie weights, and bake at 375°F for about 15 minutes. This sets the base before the wet filling goes in.
- Filling releases too much liquid: Make sure the mushrooms are fully cooked down before you add the cream — they should look dry in the pan, not wet. Excess moisture from underdrained mushrooms will pool under the filling and soften the crust.
- Crust shrinks during blind baking: Chill the shaped, unbaked crust in the tart pan for at least 20 minutes before it goes into the oven. Skipping this rest is the main reason crusts pull away from the sides.
- Filling doesn’t set and slides when sliced: The vegan cream needs enough time in the oven to firm up slightly — don’t pull the tart early just because the edges look done. The center should jiggle only slightly, like a barely-set custard, not slosh.
- Crust tears when pressing into the pan: If the dough cracks, just press it back together with your fingers. It doesn’t need to be perfect — small repairs are invisible once baked.
Substitutions that actually work
- Vegan cream: Full-fat canned coconut cream works well and adds very little coconut flavor once baked. Blended raw cashews (soaked 2 hours, drained, blended with a splash of water to a pourable consistency) is the richer option and sets slightly firmer.
- Cremini mushrooms: Shiitake caps or a mix of shiitake and cremini both work. Avoid canned mushrooms — the texture turns mushy.
- All-purpose flour in the crust: A 1:1 gluten-free baking flour blend substitutes directly. The crust will be slightly more crumbly, so handle it gently and patch any cracks before blind baking.
- Fresh thyme: Dried thyme works at one-third the quantity. Add it to the onions early so it has time to bloom in the oil. Skip the garnish of fresh herb sprigs on top — not worth the extra prep or the additional dish to wash.
Storage and reheating
Store leftover tart loosely covered in the fridge for up to 3 days. Reheat individual slices in a 350°F oven for 10–12 minutes directly on the oven rack — this keeps the crust from going soft, which reheating in the microwave will not. The tart also holds well at room temperature for up to 2 hours, making it a practical option for a buffet or potluck. To freeze, cool the tart completely, wrap individual slices in parchment and then foil, and freeze for up to 6 weeks. Thaw overnight in the fridge and reheat in the oven as above.
Caramelized Onion and Mushroom Tart
Equipment
- 10-inch tart pan with removable bottom
- Skillet or sauté pan
- Mixing bowl
- Rolling Pin
- Oven
- Parchment paper
- Pastry brush
Ingredients
For the Tart Crust:
- 1 ¼ cups all-purpose flour preferably unbleached
- ¼ tsp sea salt
- ½ cup vegan butter chilled and cut into small cubes
- 3 tbsp ice water more as needed
For the Filling:
- 2 tbsp olive oil extra virgin
- 2 large yellow onions thinly sliced
- 1 tsp organic cane sugar to help caramelize the onions
- ½ tsp balsamic vinegar
- 3 cups cremini mushrooms sliced
- 3 cloves garlic minced
- 1 tsp fresh thyme leaves or 1/2 tsp dried thyme
- ½ tsp sea salt to taste
- ¼ tsp black pepper freshly cracked
- ½ cup unsweetened vegan cream or cashew cream
Instructions
Make the Tart Crust:
- In a large bowl, whisk together flour and salt. Using your fingertips or a pastry cutter, work the chilled vegan butter into the flour until the texture resembles coarse crumbs with pea-sized bits.
- Add ice water, 1 tablespoon at a time, mixing gently until the dough just holds together. Form into a disc, wrap in parchment paper, and chill in the fridge for 30 minutes.
- Preheat oven to 375°F (190°C). On a floured surface, roll out the chilled dough into a 12-inch circle. Gently press into a 10-inch tart pan. Trim the edges, prick the base with a fork, and pre-bake for 10 minutes.
Prepare the Filling:
- Heat olive oil in a skillet over medium heat. Add sliced onions and sugar and sauté, stirring occasionally, for 25–30 minutes until deeply golden and caramelized. Stir in balsamic vinegar and cook 1 more minute.
- Push onions to the side and add mushrooms to the pan. Cook 8–10 minutes until browned, then stir in garlic and thyme and cook for another 2–3 minutes. Season with sea salt and pepper. Remove from heat.
- Spoon the caramelized onion and mushroom mixture into the pre-baked tart shell. Pour in vegan cream, allowing it to settle between the onion and mushrooms without overflowing.
- Bake at 375°F (190°C) for 20–25 minutes until the filling is set and lightly golden on top. Let cool for 10 minutes before slicing.
Notes
- For a gluten-free version, use a gluten-free flour blend with xanthan gum for the pastry crust.
- For deeper flavor, deglaze mushrooms with a splash of dry white wine before adding garlic and thyme.
- Cashew cream offers a richer, neutral flavor in the filling compared to store-bought vegan creams.
Nutrition
Common questions
Can I make this tart ahead of time?
Yes — the fully baked tart keeps well in the fridge for up to 3 days. You can also make the caramelized onion and mushroom filling up to 2 days ahead and store it separately in a sealed container, then assemble and bake the day you need it.
How do I know when the onions are actually done caramelizing?
They should be deep golden-brown, significantly reduced in volume, and taste sweet rather than sharp. If they still look pale or translucent after 20 minutes, your heat is too low — nudge it up slightly, but don’t rush past medium.
My tart pan doesn’t have a removable bottom. Can I still use it?
You can, but getting clean slices out will be harder. Line the pan with parchment before pressing in the dough, leaving enough overhang to lift the tart out after it cools slightly.
Is there a way to tell when the tart is done baking?
The crust edges should be golden and the filling should look set at the edges with only a slight jiggle in the very center when you gently shake the pan. If the surface looks wet and moves freely, give it another 5 minutes.
Can I use store-bought vegan pastry dough instead of making the crust from scratch?
Yes, and it works well here. Look for a plain shortcrust or pie-style dough — most store-bought puff pastry is accidentally vegan, and that also works, though the texture will be lighter and more layered than a traditional tart base.