Caramelized Onion Tart with Gruyere Cheese And Bacon

This savory tart comes from the Pastry Kitchen at Culinary School, where I made it many times. One of the things I love about this recipe is how the familiar flavors of French onion soup come together in a crisp pastry shell. Bacon adds savory depth, balsamic vinegar brings a sweet-tart note, and cream gives the onion filling a rich finish.

Serve this caramelized onion tart as an appetizer for a dinner party or wine tasting, or pair a slice with a tossed green salad for a light meal. You can blind-bake the tart shell ahead of time, making the recipe easier to manage when you’re entertaining.

Why You’ll Love This Tart

• Sweet and savory flavor: Slowly cooked onions, smoky bacon and balsamic vinegar create a rich filling that pairs beautifully with Gruyère.
• A creamy onion filling: Heavy cream brings the cooked onions and seasonings together, while the cheese adds a nutty, savory finish.
• A recipe with culinary-school roots: Straight from the pastry kitchen.
• A practical make-ahead option: Prepare the blind-baked tart shell in advance, then assemble and bake the tart closer to serving time.

Ingredients You’ll Need

Here are the key ingredients that give this tart its signature flavor. Refer to the recipe card for the exact measurements.

• Onions: Sweet onions or yellow onions both work. Slice them into even, ¼-inch rounds so they cook at a similar rate.
• Bacon: Six ounces of chopped bacon adds smoky, savory flavor. You’ll also use some of the rendered bacon fat to cook the onions.
• Balsamic vinegar: Adds a concentrated sweet-tart flavor that balances the sweetness of the cooked onions. It is an important part of this recipe’s flavor profile. If you enjoy the rich, tangy flavor of balsamic vinegar in savory recipes, learn How to make a balsamic reduction, for another way to add concentrated balsamic flavor to your cooking.
• Heavy cream: Stirred into the cooked onion mixture, cream gives the filling a rich texture.
• Gruyère cheese: This nutty, savory cheese melts over the onion filling and complements the bacon and balsamic vinegar.
• Thyme: Adds an herbal note that works particularly well with the onions. The recipe uses fresh or dried thyme.
• Sugar and salt: Added during the onion-cooking process to season the filling and support its flavor development.
• Pie crust: Use a blind-baked tart shell. A homemade all-butter crust is my preferred choice, and the crust can be prepared ahead of time.

Ingredient Substitutions

The original combination of bacon, balsamic vinegar, thyme, cream and Gruyère is what gives this tart its distinctive flavor. If you need to make a substitution, here are the options already established in the recipe.

• Gruyère cheese: Emmental, Swiss cheese, Comté, Fontina or Jarlsberg can be used instead. Jarlsberg is my first choice among these alternatives.
• Thyme: Fresh thyme can replace dried thyme. Fresh thyme also makes a nice garnish.
• Balsamic vinegar: I recommend keeping the balsamic vinegar in this recipe. Its rich, sweet-tart flavor is an important part of the onion filling.

How To Make This Caramelized Onion Tart

The filling is cooked before it goes into the tart shell, so the process has two main parts: caramelizing the onions and assembling the tart for its final bake.

1. Step 1: Cook The Bacon: Cut the bacon into lardons and cook it in a large frying pan, stirring often, until cooked and lightly crisp. Transfer the bacon to a plate and leave about two tablespoons of the rendered fat in the pan. The bacon fat becomes the cooking fat for the onions, carrying its savory flavor into the filling.
2. Step 2: Prepare the Onions: In the meantime, cut the ends off each onion, remove skin and slice into ¼ inch rounds.

3. Step 3: Caramelize the Onions: Reduce the heat to medium-low. Add the onions, sugar and salt to the reserved bacon fat. Stir frequently as the onions soften and cook down to about one-third of their original volume, approximately 30 minutes. If the bacon hasn’t produced enough fat, add a generous tablespoon of butter.
4. Step 4: Add the Balsamic Vinegar: Add the balsamic vinegar, reduce the heat to low, and continue cooking for about 15 minutes, until the onions are evenly golden brown and softened. Stir frequently and scrape up the browned bits from the bottom of the pan. Watch the onions carefully: the goal is soft, golden, caramelized onions, not burned edges. Lea Ann’s tip: Don’t rush this step by turning up the heat. Give the onions time to soften and develop their flavor. Slice them evenly, keep the heat low enough to prevent scorching, and use their color and texture as your guide.
5. Step 5: Finish the onion filling: Remove the pan from the heat. Stir in the thyme, freshly ground black pepper and cooked bacon. Allow the mixture to cool slightly, then stir in the heavy cream until everything is thoroughly combined. The filling is now ready for the prepared tart shell.

6. Step 6: Fill the blind-baked tart shell: Preheat the oven to 350°F. Place the onion filling in the 8-inch blind-baked tart shell. Spread it evenly with the back of a spoon so the filling reaches the edges.
7. Step 7: Add the Gruyere Cheese. Sprinkle the grated Gruyère evenly over the onion filling.

1. Step 8: Bake the tart: Place the tart on a baking sheet and bake at 350°F for about 15 minutes, until the filling is set. Allow the tart to settle briefly before removing it from the pan. Serve warm or at room temperature, garnished with a sprig of thyme.

Make-Ahead, Storage and Reheating

• Can you make this caramelized onion tart ahead of time? Yes. You can blind-bake the crust up to two days in advance. When you’re ready to prepare the tart, assemble it and refrigerate it until you’re ready to bake. You can also bake the tart a couple of hours before serving. It works well warm or at room temperature.
• Leftovers: Store cooled leftover tart in a sealed container in the refrigerator for four to five days.
• Can You Freeze This Tart? Yes. Freeze leftover slices in a freezer-safe container for up to three months. Keep in mind that the pastry’s texture may change after freezing and reheating.
• Reheating: Let a slice come to room temperature if you prefer to eat it that way, or reheat it gently in a 300°F oven. An air fryer is another option when you want to help the crust retain its crispness. I do not recommend a microwave because the crust will soften.