In a mixing bowl, combine gluten-free flour and salt. Using your fingers or a pastry cutter, incorporate the cold butter until you have coarse crumbs.

Gradually add ice-cold water, mixing until the dough just holds together. Divide into two portions, flatten into discs, wrap, and refrigerate for at least 1 hour.

In a skillet over medium heat, melt the butter and add the thinly sliced onions.

Sauté until slightly softened (about 5 minutes). Sprinkle with sugar and a pinch of salt.

Pour in the broth and scrape the flavorful bits from the pan. Lower the heat and cook, stirring occasionally, until the onions are deeply golden, about 25-30 minutes. Set aside.

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

Roll out the chilled dough on a lightly floured surface to about ⅛-inch thickness.

Cut into 4-inch rounds and gently press each round into small tart pans. Prick the bottoms with a fork and chill for about 20 minutes.

Whisk the egg with 1 tablespoon of water and brush onto the chilled crusts.

Spoon the caramelized onions into each tart shell. Layer the apple slices and brie, then sprinkle with Parmesan cheese.

Place the tarts in the oven for 20–25 minutes or until the crusts are golden and the cheese is bubbling and lightly browned.

Allow the tarts to cool slightly before topping with fresh thyme, flaky sea salt, freshly cracked black pepper, and a light drizzle of honey. Enjoy while warm!