Combine gluten-free multipurpose flour, almond flour, and kosher salt in a bowl. In a separate bowl, cream together the unsalted butter and powdered sugar until light and fluffy. Add in the egg, mixing well, then gradually add the flour mixture to form a soft dough. Wrap the dough in plastic wrap and chill in the refrigerator for at least one hour to firm up.

Peel and dice your apples, then toss them with lemon juice in a large mixing bowl. In a skillet over medium heat, sauté the apple pieces in a tablespoon of unsalted butter for about 3-5 minutes until slightly tender. Stir in brown sugar, cornstarch, cinnamon, nutmeg, and vanilla extract, cooking until the mixture thickens slightly and becomes syrupy, about 2 more minutes.

In a mixing bowl, combine the gluten-free flour, brown sugar, chopped pecans, cinnamon, and a pinch of salt. Drizzle in cold unsalted butter, using your fingers or a pastry cutter to mix until the mixture resembles coarse crumbs. Place the streusel in the refrigerator to keep it cool while you assemble the tart.

Preheat your oven to 350°F (175°C). Roll out the chilled crust on a floured surface until it's about 1/8-inch thick. Transfer the crust to a tart pan, pressing it into the edges and poking the bottom with a fork to prevent bubbles. Sprinkle a light layer of flour and sugar over the crust, then fill it with the cooked apple mixture, drizzling caramel over the top.

Slide the assembled Gluten Free Caramel Apple Tart into your preheated oven and bake for 15 minutes at 350°F. Increase the temperature to 375°F (190°C) for an additional 5-10 minutes, or until the edges are golden brown. Finally, lower the oven back to 350°F and bake for another 32-38 minutes, until the filling is bubbling and the streusel topping is lightly browned.