This onion tart is richly savory, packed with umami from caramelized onions, mushrooms, mustard, and optional roasted tomatoes. It’s easy to assemble using frozen puff pastry.
Cut into squares for a handheld appetizer, or serve with a salad for a light dinner. The filling can be made ahead—bring it to room temperature before filling the pastry and baking.
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This French onion tart with mushrooms is ideal for cool evenings. Sweet, slightly caramelized onions combine with sautéed mushrooms and a touch of Dijon for deep savory flavor. For gatherings, slice it into bite-sized squares; for dinner, pair it with a fresh salad.
Yellow onions are the preferred choice here for their balance of sweetness and body, though sweet varieties like Vidalia or Walla Walla will also work. Frozen puff pastry keeps this recipe approachable—perfect for both beginners and confident home cooks. Below you’ll find ingredient guidance, a clear overview, and step-by-step instructions with photos.
🧅 Ingredient Tips
- Frozen puff pastry – I prefer DuFour, but Pepperidge Farm works well too.
- Onions – Yellow onions are ideal. Sweet onions (Vidalia, Walla Walla) are acceptable. Avoid red onions for this tart.
- Mushrooms – White button mushrooms are fine; mix varieties if you want more depth.
- Mustard – Dijon or grain mustard adds the right savory note; skip classic yellow mustard.
- Tomatoes (optional) – Halved grape or cherry tomatoes roasted with balsamic make a great topping. They can be prepared in advance and refrigerated.
📝 Instructions Overview
Full directions are in the recipe card below. Here’s a quick summary of the process.
Step 1
Roll and shape the pastry. Thaw puff pastry per package directions and lightly roll to even the surface. Trim one-inch strips from the sides and place them on the sheet to form a raised crust frame. Partially bake until lightly golden, then reduce oven temperature for the final bake later.
Step 2
Sauté the vegetables. Caramelize the sliced onions in butter, cooking low and slow for the best sweetness. Sauté finely chopped mushrooms separately until nicely browned, then combine with the onions and thyme.
Step 3
Assemble the tart. Spread a thin layer of Dijon mustard on the pre-baked pastry, add the onion and mushroom mixture, and top with roasted tomatoes if desired.
Step 4
Final bake and serve. Return the filled tart to the oven for a short final bake, then finish with fresh thyme and serve warm. You can prepare the filling up to a week ahead for easy assembly.
Pro tips:
Slice onions by halving them and cutting lengthwise so they lay flatter in the pan. Buy pre-sliced mushrooms if you prefer—stack and slice for even pieces. Once the onions begin to brown, avoid frequent stirring so they caramelize properly; do the same with the mushrooms.
Step 3 (optional)
Slow-roast cherry tomatoes.
Halve cherry tomatoes, toss with oil and a splash of balsamic, and roast at 350°F for about 45 minutes. They add bright, concentrated flavor and can be stored refrigerated for up to two weeks.
Step 4 (15 minutes)
Final bake. Spread a thin layer of Dijon across the tart base, spoon in the filling, and add roasted tomatoes if using. Bake about 10 minutes, then finish with fresh thyme before serving.
Step 5
Serve. Slice into squares of your preferred size and serve warm.
Timing note: While puff pastry thaws and bakes you can prepare the onions, mushrooms, and tomatoes. The onions and mushrooms can cook during the pastry’s pre-bake so the final assembly is quick.
For another savory option, try a roasted cauliflower and kale tart baked in a cheddar crust.
5 Ingredient Onion Tart with Mushrooms Recipe
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Equipment
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rolling pin
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Chef’s knife
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Sheet pan
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Large skillet
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Medium skillet
Ingredients
- 1 frozen puff pastry
- 9 roasted cherry tomato halves or sun-dried tomatoes (optional)
- 2 ½ pounds yellow onions, about 6 cups when thinly sliced
- 3 tablespoons unsalted butter, divided
- ¾ teaspoon kosher salt, divided (or half this amount with table salt)
- 1 cup finely chopped white mushrooms
- 8 twigs fresh thyme, divided
- 2 teaspoons Dijon mustard
Instructions
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Preheat the oven to 400˚F. Thaw the puff pastry according to package directions and carefully unfold. Lightly roll to smooth any creases.
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Trim a one-inch strip from each side. Moisten the edges slightly with water and place the strips atop the pastry to create a raised frame for the tart crust.
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Place the tart on a parchment-lined baking sheet and prick the bottom several times with a fork so steam can escape. Bake until lightly browned, about 25–30 minutes. If the bottom puffs, prick and gently press it down; the filling will weigh it down during the final bake. Allow to cool.
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While the pastry bakes, thinly slice the onions. Melt 2 tablespoons butter in a large skillet over medium heat, add the onions and ½ teaspoon salt, and cook, stirring occasionally, until softened and caramelized—about 45 minutes. When browning begins, lower heat and leave them undisturbed more often to develop color.
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Chop the mushrooms and sauté in a separate medium pan with 1 tablespoon butter and half the thyme until golden. Remove the thyme sprigs, then add the mushrooms to the onions with the remaining salt. Tip: move mushrooms only occasionally so they brown well.
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When the tart has cooled, spread a thin layer of Dijon mustard across the base to boost savory flavor.
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Lower the oven to 350˚F. Fill the pre-baked pastry with the onion and mushroom mixture and arrange roasted tomatoes on top if using. Bake for 10 minutes, then sprinkle with remaining fresh thyme and remove from the oven.
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Serve warm, sliced into squares.
