Step 1: Thaw the puff pastry according to package directions, typically in the refrigerator overnight or at room temperature for about 30 minutes. Preheat your oven to 400°F (200°C).
Step 2: Gently unroll the thawed puff pastry and carefully press it into a 9-inch tart pan with a removable bottom, allowing any excess to drape over the edges. Prick the bottom of the pastry several times with a fork to prevent excessive puffing. Place a piece of parchment paper over the pastry and fill with pie weights or dried beans. Blind bake for 15 minutes. Remove the weights and parchment, then continue to bake for another 5-7 minutes, until lightly golden. Carefully trim any excess pastry from the edges and set aside to cool slightly. Reduce oven temperature to 375°F (190°C).
Step 3: While the pastry bakes, begin caramelizing the onions. In a large skillet or Dutch oven, melt the butter over medium-low heat. Add the thinly sliced onions and 1/2 teaspoon of salt. Cook, stirring occasionally, for 30-45 minutes (or up to 1 hour), until the onions are deeply golden brown, very soft, and sweet. If they start to stick to the pan, add a small splash of water or olive oil to deglaze. Stir in the 1/4 teaspoon of black pepper.
Step 4: Once the pastry shell has cooled slightly and the onions are caramelized, spread the Dijon mustard evenly over the bottom of the blind-baked pastry shell. This adds a nice tang and barrier.
Step 5: Sprinkle half of the grated Gruyere cheese evenly over the mustard layer.
Step 6: Spoon the caramelized onions over the cheese, spreading them out to form an even layer.
Step 7: Top the onions with the remaining grated Gruyere cheese. If using, sprinkle with fresh thyme leaves.
Step 8: Bake the tart in the preheated 375°F (190°C) oven for 25-30 minutes, or until the pastry is golden brown and the cheese is bubbly and melted, with a beautiful golden crust.
Step 9: Remove the tart from the oven and let it rest for at least 10 minutes before carefully removing it from the tart pan. Slice and serve warm or at room temperature.