• 2 large onions, thinly sliced
• 3 tablespoons butter, divided
• 1/2 cup Champagne
• 8 ounces mixed mushrooms of choice, sliced
• 1 tablespoon fresh thyme leaves
• 3 cloves garlic, minced
• 1 (17-ounce) package puffed pastry, thawed in refrigerator
• 1/4 cup Dijon mustard, divided
• 8 ounces Gruyere cheese
• 1/4 cup grated Parmesan
• 1 egg, whisked for egg wash
• Salt and pepper, to taste

1. Preheat oven to 400 degrees.
2. In a heavy-bottom skillet over medium-low heat, caramelize onions with 2 tablespoons butter for about 30 minutes, stirring occasionally. Turn heat to medium-high and add Champagne; stir. Continue cooking until liquid is absorbed and onions are golden brown and jammy. Season with salt and set aside to cool.
3. Add mushrooms and remaining butter to skillet. Cook over medium-high heat, stirring gently until golden brown and softened, about 5 minutes. Stir in thyme, minced garlic and season with salt and pepper, set aside.
4. Line two baking sheets with parchment. Unfold both sheets of puff pastry and lightly score a ½-inch border around edges, leaving a light mark. Prick the center area with a fork. Spread 2 tablespoons mustard on each pastry and transfer to the baking sheets. Evenly divide Gruyere cheese, caramelized onions, mushrooms and Parmesan cheese, leaving space at the border.
5. Brush border of tarts with egg wash and bake for 25 to 30 minutes or until pastry is puffed and edges are golden brown. Let cool 10 minutes before cutting tart and topping with more thyme, if desired.

Options: Onions and mushrooms can be made ahead of time before adding to thawed puff pastry. Tart can be baked ahead and frozen. To reheat, bake for 15 to 20 minutes at 350 degrees.

Yield: 2 tarts/24 servings