Place ½ cup of the oats in a food processor. Pulse until the mixture is very fine and almost flour-like.
½ cup old fashioned oats
Combine the rest of the oats, the pureed oats, walnuts, brown sugar, cinnamon, and melted butter together.
¾ cup old fashioned oats, ¼ cup walnuts, ¼ cup brown sugar, ¼ cup butter, ½ teaspoon ground cinnamon, ⅛ teaspoon salt
Filling
Add sugar and cornstarch to the raspberries. Toss lightly to coat.
4 cups raspberries, ⅓ cup granulated sugar, 1 tablespoon cornstarch
Place raspberries in a lightly greased 8x8-inch baking dish.
Top with oat mixture.
Bake in a preheated 350°F oven for 35-45 minutes until lightly browned and bubbly.
Allow crisp to cool. Serve warm or at room temperature.
Notes
The raspberry filling has just one tablespoon of cornstarch added to it. It helps thicken the filling just slightly, but it is still very juicy. If you like a thicker filling (more pie-like) add an additional tablespoon of cornstarch.
The crisp is done when the topping is nicely browned and the filling is bubbly all over.
Crisp is best served warm or at room temperature.
Any leftovers can be covered and stored at room temperature for one day.
To use frozen raspberries, do not thaw berries. Increase baking time by 10 minutes.