You can make these shells in batches since each one requires two pans.
Place flour, salt, and walnuts in the bowl of a food processor. Pulse until the walnuts are small, but not fine. Add butter, and pulse until the mixture resembles small peas, about 15 seconds.
With the machine running, add 1/4 cup ice water through the feed tube. Pulse until dough just starts to come together when you press it with your fingers. If crumbly, add up to 2 more tablespoons water, 1 tablespoon at a time.
Form dough into a ball. Flatten into a disk, and wrap in plastic. Transfer to the refrigerator, and chill at least 1 hour.
Set twenty-four 2-inch tartlet pans on a baking sheet. Lightly dust a clean work surface with flour. Roll out dough to an 1/8-inch thickness. Using a paring knife, cut the pastry into twenty-four squares slightly larger than the pans. Press dough into pans, and trim overhanging dough. Place a second tartlet pan on top of each lined pan, weighing down pastry. Chill 30 minutes more.
Heat oven to 375 degrees. Bake shells until lightly browned on the edges, about 10 minutes. Remove top pans, and continue baking until cooked through and browned all over, 12 to 15 minutes more. Turn out shells, and transfer to wire racks to cool. Store shells in an airtight container up to 3 days.