TO MAKE THE CRUST Combine the hazelnuts, flour, salt, and sugar in a food processor fitted with the metal blade.
Add the butter and shortening and pulse until the mixture resembles coarse meal.
Add the sour cream and water, and process for a few seconds, just until a ball of dough begins to form.
Do not overprocess. (To make the dough by hand, place the dry ingredients in a large bowl, and use a pastry blender to cut the butter and shortening into the flour mixture.
Add the sour cream and ice water, and mix just until it comes together and forms a mass.) Transfer the dough to a floured work surface, gathering all the loose bits, and form into a disk about 1 inch thick.
Wrap the dough in plastic wrap, and refrigerate for at least 30 minutes, or overnight.
Preheat the oven to 425°F.
Have ready a 10-inch tart pan with a removable bottom.
On a lightly floured work surface, roll out the dough into a circle about 12 inches in diameter.
Dust the work surface and dough with a little more flour, as necessary, to keep the dough from sticking.
Roll the dough around the rolling pin, lift it over the tart pan, and unroll the dough over the pan.
Adjust to center the dough; then press it into place.
Trim the overhang to 1/2 inch and fold in to reinforce the side.
Freeze the tart shell for 20 minutes. (The tart shell can be made up to this point and frozen for up to 1 month.)