The essence of childhood is truly present in this stripped-down summer dessert: tender and tart, crumbly, and just a tiny touch sweet. Don't skip the generous scoops of ice cream right on top. Check all formalities for the night: Pass the spoons and dig straight in.
1. Preheat the oven to 375 degrees F.
2. To make the topping: Combine the flour, oats, sugar, and salt in a bowl and stir together. Toss the melted butter into the flour mixture until the consistency resembles coarse breadcrumbs. Add half the nuts and pinch together to make a tight dough.
3. To make the filling: Combine the rhubarb, raspberries, sugar, and cornstarch in a bowl and toss to combine. Transfer to a 2-qt. round or oval casserole dish or deep-dish pie plate. Top with the crumble topping, letting a bit of the fruit peek out the sides. Sprinkle with the remaining nuts and bake until golden and bubbling, 30 to 40 minutes. Serve warm, with ice cream scooped onto the top, a drizzle of fresh, cold cream, and a stack of spoons.