1. Preheat the oven to 375įF. Lightly butter a deep-dish pie plate or other shallow 1 1/2- to 2-quart nonreactive baking dish, and set it aside.
2. Combine the rhubarb, strawberries, sugar, cornstarch, and lemon juice in a medium mixing bowl and toss to combine. Transfer the mixture to the prepared baking dish, and set it aside while you prepare the topping.
3. Combine all the remaining ingredients in the bowl of an electric mixer fitted with the paddle attachment, and process on low speed until the mixture is crumbly and coarse.
4. Sprinkle the topping over the fruit, and then place the baking dish on a rimmed baking sheet (to catch any juices that may bubble over). Transfer it to the oven and bake until the topping is golden brown and crisp, and the juices are bubbly and glossy, 40 to 45 minutes. Set aside to cool briefly, then serve hot or warm, with a scoop of vanilla ice cream if desired.
6 to 8 servings