• 3 large yellow onions: Yellow onions are the workhorse of caramelization. Their balanced sweetness and savory depth make them ideal for this process. Look for firm onions without blemishes. About 2 pounds in total is a good starting point.
• 2 tablespoons olive oil: Use a good quality extra virgin olive oil. It adds flavor and helps the onions caramelize beautifully without burning. You can also use butter or a combination of butter and olive oil for a richer flavor.
• 1 tablespoon butter (unsalted): Butter adds richness and helps in the browning process of the onions. Unsalted butter allows you to control the overall saltiness of the dish.
• 1 teaspoon granulated sugar: A touch of sugar assists in the caramelization process, enhancing the natural sweetness of the onions and promoting browning. You can also use brown sugar for a slightly molasses-like note.
• 1/2 teaspoon balsamic vinegar: Balsamic vinegar adds a crucial layer of acidity and depth to the caramelized onions, cutting through the sweetness and adding complexity. Use a good quality balsamic for the best flavor.
• 1/2 teaspoon dried thyme (or 1 teaspoon fresh thyme leaves): Thyme provides a subtle earthy and herbaceous note that complements the sweetness of the onions and the richness of the cheese. Fresh thyme is always preferable if available, but dried thyme works well too.
• Salt and freshly ground black pepper: Seasoning is key! Salt enhances the flavors of the onions and balances the sweetness. Freshly ground black pepper adds a touch of spice and complexity. Adjust the amount of salt and pepper to your taste throughout the caramelization process.
• 8 ounces cream cheese, softened: Cream cheese forms the creamy and tangy base of the cheese filling. Ensure it’s softened to room temperature for easy mixing and a smooth texture. Full-fat cream cheese provides the best richness and flavor.
• 4 ounces Gruyère cheese, grated: Gruyère is a classic choice for onion tarts. It’s a nutty, slightly salty cheese that melts beautifully and complements the caramelized onions perfectly. You can substitute with other cheeses like Swiss, Fontina, or even a sharp cheddar for a different flavor profile.
• 2 ounces Parmesan cheese, grated: Parmesan cheese adds a salty, umami-rich depth to the filling. Use freshly grated Parmesan for the best flavor and texture.
• 1 large egg: The egg acts as a binder, holding the cheese filling together and giving it a slightly richer texture.
• 1/4 cup heavy cream or crème fraîche: Heavy cream or crème fraîche adds extra richness and creaminess to the cheese filling. Crème fraîche will provide a slightly tangier note. You can substitute with whole milk for a lighter option, but the texture will be less rich.
• Pinch of nutmeg (optional): A very small pinch of nutmeg adds a warm, subtle spice note that enhances the flavors of the cheese and onions. Be careful not to overdo it, as nutmeg can be overpowering.
• 1 box (14.1 ounces) refrigerated pie crusts (or homemade pie crust): For convenience, refrigerated pie crusts work wonderfully for this recipe. They are readily available and provide a flaky, buttery base. You can also use puff pastry for a different texture or make your own homemade pie crust for a truly from-scratch experience. Using two crusts will provide enough for the top and bottom of the tarts or for multiple smaller tarts.
• Fresh thyme sprigs: For a beautiful and aromatic garnish.
• Balsamic glaze: For drizzling over the finished tarts, adding a touch of sweetness and tanginess.
