In a large mixing bowl, mix the flour, brown sugar, cinnamon, and salt. With a food processor, pastry blender or your fingers, work the butter into the flour mixture just until it comes together and large clumps form. Fold nuts into the mixture. Chill until ready to use.
Cook's Note: Crisp topping can be refrigerated up to a week or frozen for up to 2 months.
Fruit Crisp Variations:
Strawberry-Rhubarb Crisp: Chop 2 pounds rhubarb into 1/2-inch thick slices. Combine with 1 pound quartered strawberries, 1/3 cup sugar and 1 tablespoon fresh lemon juice. Allow mixture to macerate for 20 minutes before transferring to a baking dish and topping with crisp topping.
Raspberry-Peach Crisp: Peel 5 large peaches and slice into wedges. Combine with 1 1/2 cups raspberries, 1 teaspoon fresh orange zest, and 1/3 cup sugar. Top the mixture with crisp topping.
Cranberry-Pear Crisp: Peel, core, and dice 4 large pears. Combine with 1 cup cranberries, 1 tablespoon fresh lemon juice, and 1/3 cup sugar. Top the mixture with crisp topping.