In a large bowl, whisk together the flour and salt. Add the cold, cubed butter and cut it into the flour using a pastry blender, your fingertips, or a food processor until the mixture resembles coarse meal with some pea-sized butter pieces.

Gradually add the ice water, 1 tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix. Form the dough into a flat disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes, or up to 2 days.

In a large heavy-bottomed skillet or Dutch oven, melt the 2 tbsp unsalted butter with the olive oil over medium-low heat. Add the thinly sliced onions, 1/2 tsp salt, and 1/4 tsp black pepper. Stir to coat the onions evenly.

Cook the onions slowly for 45-60 minutes, stirring occasionally. The key is low heat and patience. The onions will soften, release their liquid, and gradually turn a deep golden brown, becoming incredibly sweet and tender. If they start to stick, add a tablespoon of water or broth.

Once deeply caramelized, stir in the balsamic vinegar (if using) and chopped fresh thyme. Cook for another 2-3 minutes, then remove from heat and set aside to cool slightly.

Preheat your oven to 375°F (190°C). On a lightly floured surface, roll out the chilled tart dough into a 12-inch circle, about 1/8 inch thick. Carefully transfer the dough to a 9-inch tart pan with a removable bottom. Press the dough gently into the bottom and up the sides. Trim any excess dough from the edges.

Line the tart shell with parchment paper and fill with pie weights or dried beans. Blind bake for 15 minutes. Remove the parchment paper and weights, then continue to bake for another 5-7 minutes, or until the crust is lightly golden. This helps prevent a soggy bottom. Remove from oven.

Spread the cooled caramelized onions evenly over the bottom of the par-baked tart crust. Sprinkle half of the shredded Gruyère cheese over the onions.

In a medium bowl, whisk together the heavy cream, eggs, remaining 1/2 cup Gruyère cheese, 1/4 tsp salt, pinch of black pepper, and pinch of nutmeg until well combined. Carefully pour this custard mixture over the onions and cheese in the tart shell.

Place the tart pan on a baking sheet (to catch any potential spills) and bake for 30-35 minutes, or until the custard is set and golden brown around the edges. A knife inserted into the center should come out clean.

Remove the tart from the oven and let it cool on a wire rack for at least 15-20 minutes before removing the outer ring of the tart pan. Cooling allows the custard to fully set.

Slice and serve warm or at room temperature. It pairs wonderfully with a simple green salad.