A buttery oat topping blankets a sweet-tart mix of strawberries and rhubarb. This easy crisp comes together fast and bakes up bubbly and golden—perfect with a scoop of vanilla ice cream.
Preheat oven to 350°F. Lightly butter or spray a 9-by-9-inch baking dish (or a 2-quart casserole).
Make the filling: In a large bowl, toss rhubarb, strawberries, granulated sugar, cornstarch, lemon juice, vanilla, and a pinch of salt until the fruit is evenly coated. Spread into the prepared dish.
Make the topping: In another bowl, combine oats, flour, brown sugar, cinnamon, and salt. Cut in the cold butter with a pastry cutter or fingers until clumps form and no dry spots remain. Stir in pecans, if using.
Crumble the topping evenly over the fruit, squeezing some pieces into larger clumps for extra crunch.
Bake until the topping is golden and the filling is bubbling around the edges, 40 to 45 minutes. If the top browns too quickly, tent loosely with foil during the last 10 minutes.
Cool for 15 minutes to thicken the juices. Serve warm, optionally with vanilla ice cream or whipped cream.
Notes
Tip: If using frozen fruit, do not thaw; increase cornstarch to 3 tablespoons. Store leftovers covered in the refrigerator for up to 3 days. Reheat in a 350°F oven for 10 to 15 minutes to re-crisp the topping.