Flaky buttery tart shells filled with sweet caramelized onions, earthy mushrooms, and melty Gruyère cheese — these savory tartlets make a show-stopping appetizer or elegant brunch bite. With a delicate crust and rich, savory filling, they strike the perfect balance between rustic charm and refined flavor.

Why You’ll Love This Recipe

These tartlets deliver deep, savory flavors in a petite, elegant package. The slow-cooked caramelized onions and mushrooms blend beautifully with the creamy Gruyère and custard base, while the homemade tart shells add a satisfying crunch. Ideal for entertaining, brunch, or a light lunch, they are as impressive in taste as they are in appearance.

Ingredients

(Tip: You can find the complete list of ingredients and their measurements in the recipe card below.)

For the Tart Shells:
all-purpose flour
unsalted butter, chilled and cubed
salt
ice water

For the Filling:
olive oil
butter
yellow onion, thinly sliced
cremini or button mushrooms, sliced
fresh thyme leaves
salt and pepper
heavy cream
large egg
shredded Gruyère cheese

Directions

1. In a food processor, pulse the flour, butter, and salt until the mixture resembles coarse crumbs. Add ice water one tablespoon at a time until the dough comes together. Wrap in plastic wrap and chill for 30 minutes.
2. Preheat oven to 375°F (190°C).
3. Roll out the chilled dough and press it into four small tartlet pans. Prick the bottoms with a fork and pre-bake the shells for 10 minutes. Set aside.
4. In a skillet, heat olive oil and butter over medium-low heat. Add the sliced onions and cook slowly, stirring often, for 20–25 minutes until deeply caramelized.
5. Add the mushrooms and thyme, and continue to cook for 6–8 minutes until softened. Season with salt and pepper.
6. In a separate bowl, whisk together the heavy cream and egg until smooth.
7. Evenly divide the mushroom-onion mixture among the tart shells. Sprinkle each with shredded Gruyère, then pour the cream mixture over the top.
8. Bake the tartlets for 20–25 minutes, or until the filling is set and golden on top.
9. Let cool slightly and garnish with fresh thyme sprigs before serving.

Servings and timing

Servings: Makes 4 tartlets
Prep Time: 30 minutes
Cook Time: 55 minutes (includes pre-baking and filling baking time)
Chill Time: 30 minutes
Total Time: About 1 hour 55 minutes

Variations

• Mini Tartlets: Use a mini muffin tin for bite-sized versions perfect for cocktail parties.
• Add Bacon: Crumble in cooked bacon with the onion and mushroom mixture for extra richness.
• Cheese Swap: Try Swiss, Fontina, or aged white cheddar in place of Gruyère.
• Gluten-Free: Use a gluten-free tart shell recipe or ready-made gluten-free tart crusts.
• Vegetarian Option: Already vegetarian, but you can enhance flavor with a splash of balsamic reduction or truffle oil.

Storage/Reheating

Store any leftover tartlets in an airtight container in the refrigerator for up to 3 days. To reheat, place in a 325°F (165°C) oven for 10–15 minutes until warmed through and the crust is crisp again. Avoid microwaving to maintain texture.

FAQs

Can I make the tart dough in advance?

Yes, the dough can be made up to 2 days in advance and kept refrigerated, or frozen for up to a month.

Can I use store-bought tart shells?

Absolutely. Pre-made tart shells can be a time-saver and still produce delicious results.

How thin should I slice the onions?

Slice them very thinly (about 1/8 inch) to ensure they caramelize evenly and blend well with the filling.

What’s the best mushroom variety for this recipe?

Cremini or button mushrooms work well, but feel free to mix in shiitake or portobello for added depth.

Can I make this recipe into a full-size tart?

Yes, use a 9-inch tart pan and increase the baking time by 5–10 minutes to ensure the filling sets properly.

Is it necessary to blind bake the crust?

Yes, partially baking the tart shells prevents a soggy bottom when the filling is added.

What wine pairs well with these tartlets?

A crisp white like Sauvignon Blanc or a light red like Pinot Noir complements the rich, savory flavors beautifully.

Can I freeze the baked tartlets?

Yes, wrap tightly and freeze for up to 1 month. Reheat in the oven for best results.

What can I use instead of heavy cream?

Half-and-half can be substituted, though the filling may be slightly less rich.

How do I prevent the crust from shrinking?

Chill the dough thoroughly before baking and avoid stretching it when pressing into the pan.

Conclusion

Caramelized Onion and Mushroom Gruyère Tartlets are the epitome of refined comfort food. With layers of complex flavor and a satisfying crunch from the homemade crust, they are well-suited for entertaining or enjoying as a sophisticated snack. Whether served warm or at room temperature, these savory tartlets are sure to impress every time.

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