Ingredients

• 2 tablespoons olive oil
• 2 tablespoons butter
• 1 large white onion, sliced
• 1 clove garlic, minced
• 1/4 cup white wine
• 1/2 cup beef broth
• 1 teaspoon dried thyme
• 1/2 teaspoon kosher salt (such as Diamond Crystal®)
• 1/4 teaspoon ground black pepper
• 1 sheet puff pastry
• 1/2 cup grated Parmesan cheese
• 1 cup shredded mozzarella cheese

Instructions

1. Preheat oven to 400 degrees F (200 degrees C).
2. Heat olive oil and butter together in a skillet over medium-high heat until the butter is melted, 2 to 3 minutes. Saute onion and garlic in the hot oil mixture until the onion begins to caramelize, 15 to 20 minutes.
3. Pour white wine over the onion mixture; bring to a simmer and cook until fragrant, about 2 minutes. Add beef broth and thyme; cook until the liquid reduces in volume by half. Season the mixture with kosher salt and black pepper.
4. Trim puff pastry and fit into a tart pan. Score a rim along the edge of the pastry. Puncture the center of the pastry with a fork. Spread the onion mixture over the pastry; top with Parmesan cheese and mozzarella cheese.
5. Bake in preheated oven until the crust is golden brown, 15 to 20 minutes.