Be careful while pulsing this pate brisee in your food processor -- the walnuts in this dough should be in small pieces that will lend texture to the crust.
Place the flour, salt, sugar, and walnuts in the bowl of a food processor. Pulse until walnuts are small, but not fine, about 10 seconds. Add butter, and pulse until mixture resembles small peas.
With machine running, add 1/4 cup ice water through feed tube. Pulse until dough just starts to come together when you press it with your fingers, about 45 seconds. If necessary, add up to 2 more tablespoons ice water. Divide into disk; wrap in plastic. Chill at least 1 hour. Dough may be frozen up to 1 month.