In a large bowl, mix oats, flour, walnuts, brown sugar, cinnamon, and salt. With your fingers or a pastry blender, rub or cut butter into oat mixture until coarse crumbs form. Cover and chill.
Rinse rhubarb; trim off and discard green parts of stalks. Cut red parts into 1/2-inch-thick slices; you need 2 cups. In a large bowl, combine granulated sugar, cornstarch, raspberries, and rhubarb and mix gently to coat. Pour into a 8×8 baking dish or shallow 2- to 3-quart baking dish and sprinkle evenly with topping.
Bake in a 375° regular or convection oven until topping is golden brown and fruit is bubbling, about 40 -45 minutes. Serve warm or at room temperature.