These recipes are from "Entertaining," by Martha Stewart.
Put flour and salt in bowl of food processor. Cut the butter and margarine into small bits and add to the flour. Process 10 seconds, or until mixture resembles coarse meal. Add the water little by little, processing as little as possible (10 to 15 seconds). Dough should just hold together in the bowl, but not be wet or sticky. Turn out onto plastic wrap, press into a ball, and chill at least 1 hour before using.
To form tart or tartlet shells, use twice as many pans as projected number of tart or tartlets. Lightly butter or spray with vegetable oil the insides of half the pans.
Roll half the pastry onto a lightly floured board to slightly less than 1/8-inch thick. Cut the pastry slightly larger than the pans, press into pans, and cut away excess with your thumb. Repeat process with rest of pastry. In the case of a large tart, cut the pastry large enough so that there will be a 1/2-inch overhang.
Press the unbuttered pans on top of each of the pastry-filled pans. These act as weights and will prevent shrinkage and puffing while baking. Line large tarts with aluminum foil and weight with beans or rice.
Chill tart or tartlets on baking sheets until ready to bake. This may be done the day before. Do not leave unbaked pastry in the tart shells longer than this or discoloration may occur. Any unused pastry can be tightly wrapped and chilled (up to 2 days) or frozen.
Preheat oven to 375 degrees. Bake the shells for 15 to 17 minutes. Press down any pans that puff up. When pastry seems to be coloring very slightly around edges, remove top pans and return pastry to oven to dry out. Do not brown. Cool the shells on racks. If not to be used immediately, store in a tightly covered plastic container in the freezer. Thaw before using.