TO MAKE THE CRUST

1. In the bowl of a food processor fitted with the blade, combine flour, sugar, xanthan gum, salt, and baking powder; pulse a few times to blend.
2. Add butter and mascarpone and pulse until the mixture resembles coarse gravel; add water and pulse until the dough just begins to come together in a ball.
3. Remove the dough onto a plastic sheet and press it together into a ball; wrap tightly and chill for at least one hour or overnight.

TO BAKE THE CROSTATA

1. Preheat the oven to 400°F (204°C).
2. Roll out three-quarters of the dough to an 11-inch circle; transfer to an ungreased fluted 9 ½-inch tart pan. Ease the dough into the pan, covering the entire surface including the corners and edges. Run the rolling pin across the edges of the pan to remove any excess dough.
3. Spread the jam evenly across the unbaked crust with a spoon.
4. Roll out the remaining dough to a 10-inch long rectangle. Use a pastry wheel or sharp knife to cut ½-inch strips of dough. Arrange strips of dough across the tart horizontally, leaving ½-inch space between them, then rotate the pan slightly and place the strips of dough on top of the first strips at a 45-degree angle.
5. Bake for 25 to 30 minutes until the filling is bubbly and the crust is browned.
6. Let the tart cool completely before serving.