Preheat oven to 350°. Combine oats, flour, walnuts, brown sugar, cinnamon, ginger, and salt in a large bowl. With your fingers, rub butter into oat mixture until blended and coarse lumps form. Cover and chill.
Combine granulated sugar and cornstarch in another large bowl. Add raspberries and rhubarb and toss gently to combine with cornstarch mixture. Pour into a shallow 2- to 3-qt. baking dish and sprinkle evenly with topping.
Bake crisp until topping is golden brown and fruit is bubbling, about 45 minutes. Serve warm or at room temperature, with ice cream.