1-First Step: Caramelize the Onions Halve and thinly slice 4 medium yellow onions. Melt 2 tablespoons unsalted butter in a large skillet over medium heat. Add onions along with 2 bay leaves and 4 thyme sprigs. Cook slowly, stirring frequently, for at least 40 minutes until onions are golden and sweet, adding small amounts of water as needed to prevent burning.

2-Second Step: Prepare the All-Butter Pastry In a mixing bowl, combine 1¼ cups all-purpose flour, 1 teaspoon sugar, and ½ teaspoon sea salt. Cut 8 tablespoons cold unsalted butter into ½-inch cubes and work into the flour mixture until it resembles coarse crumbs. Gradually add ⅓ cup ice water, mixing gently until dough comes together. Shape into a rectangle, wrap in plastic, and chill for at least 1 hour to improve texture.

3-Third Step: Roll and Assemble the Tart Roll chilled dough into a 12 by 15-inch rectangle on parchment paper. Spread 1 tablespoon ground Dijon mustard thinly over the surface, leaving about a 2-inch border. Mix caramelized onions with ½ cup grated Gruyere and ½ cup grated Parmesan cheese. Evenly distribute this filling over the dough.

4-Fourth Step: Fold and Brush the Edges Fold the 2-inch border edges over the filling and press to seal gently. Brush the exposed dough edges and top of tart with 1 lightly beaten egg for a golden finish. Scatter fresh herbs like thyme or parsley on top and add a light dusting of freshly grated nutmeg or star anise for subtle aromatic complexity.

5-Final Step: Bake and Serve Preheat oven to 400°F (200°C). Bake the tart in the middle rack for 20 to 25 minutes until it is puffed, golden, and bubbly. Remove and let rest for about 30 minutes for the filling to set before slicing. Serve warm or at room temperature, perhaps beside a crisp green salad for a complete meal.