In a mixing bowl, combine the gluten-free flour and salt. Add the cold butter, working it into the flour until it resembles coarse crumbs.

Gradually add ice-cold water, mixing gently until the dough comes together. Divide into two portions, flatten into discs, wrap in plastic, and chill for at least 1 hour.

Melt the butter in a skillet over medium heat. Add the sliced onions, cooking until softened (about 5 minutes).

Sprinkle them with sugar and a pinch of salt, pour in the broth, and reduce the heat to medium-low.

Continue cooking for 25–30 minutes, stirring occasionally, until they are deeply golden and caramelized. Set aside.

Preheat the oven to 400°F. Line two baking sheets with parchment paper.

On a lightly floured surface, roll out the chilled dough to about 1/8-inch thickness. Cut out 4-inch rounds and fit them into small tart pans, folding the edges inward if needed.

Prick the bottoms with a fork and chill for 20 more minutes.

Whisk the egg with 1 tablespoon of water and brush it lightly over the crusts.

Divide the caramelized onions among the tart shells, layering apple slices and brie on top. Finish with a sprinkle of Parmesan cheese.

Bake for 20–25 minutes, until the crusts are golden and the cheese is bubbly and lightly browned.

Allow the tarts to cool slightly, then sprinkle with fresh thyme, flaky sea salt, and freshly ground black pepper. Drizzle with honey before serving.