Place the flour, starches and xanthan gum (or all purpose gluten-free flour) in the bowl of a food processor along with the salt. Pulse a few times to combine. Add the butter and process until the mixture looks like coarse meal with a few larger pieces of butter in it. With the machine running add the water, a little at a time, just until the dough starts to form a ball around the blades. The dough should be soft but not sticky.
Gather the dough into a ball and wrap with plastic wrap. Flatten the ball into a disk and refrigerate for an hour. Can be made several days ahead at this point and stored in the fridge. If refrigerated for more than an hour, remove the dough from the fridge about 10 minutes before rolling to let it soften a bit.
Spray a 10 or 11 inch tart pan with removable bottom lightly with gluten-free, non-stick cooking spray.
Roll the dough between two pieces of parchment paper or plastic wrap to about ¼ of an inch thick and a little larger than your tart pan. Remove the top piece of paper and flip the dough into the tart pan then remove the other sheet of paper. Gently push the dough into the pan to fill it and remove any excess. If the dough splits, just push it back together.
Preheat oven to 350 degrees and prepare the filling.
Crumble the almond paste into the bowl of a food processor and pulse several times to break it up. Add the powdered sugar, melted butter and eggs. Process until the mixture is smooth. Pour the mixture into the tart crust and smooth the top with a spatula.
Peel the pears, cut them in half and remove the core and the bottom end with a melon-baller or paring knife. Lay each pear half flat on a cutting board, cut side down, and cut into ½ inch slices. Gently push the slices down with your hand to fan them out a bit. Slide a large spatula under the sliced pear half and place it on the filling with the top of the pear half facing the center of the tart. Repeat with remaining pear halves. Brush the pears with the lemon juice and sprinkle the granulated sugar on top.
Bake for 40 – 45 minutes or until the filling is set. Brush the tops of the pears with melted jelly if desired. Let cool in the tart pan them remove the outer ring and serve.
*Or you can substitute 1¼ cups of all purpose gluten-free flour for the above ingredients.