Preheat your oven to 350°F and butter a 13"x9" baking dish.
Prepare the topping by mixing 1.5 cups all-purpose flour, 1 cup rolled oats, 1 cup brown sugar, 1 cup softened butter, and zest from 1 orange and set aside.
In a large bowl, toss together 1 pound rhubarb, 4 cups strawberries, ½ cup granulated sugar, 2 tablespoons cornstarch, and juice from 1 fresh orange until everything is evenly coated. The cornstarch is what keeps this from turning into a soggy mess, so don't skip it. Pour the filling into your prepared baking dish and spread it out evenly.
Scatter the crisp topping evenly over the fruit filling —I like to use a cookie scoop for uniform pieces and because my kids are crisp monsters!
Bake for 45 minutes or until the topping is deeply golden and the fruit filling is bubbling up around the edges. If it's browning too fast, tent loosely with foil for the last 10 minutes.
Cool for 15 minutes to let filling set, then serve warm.
Serve warm with vanilla ice cream or whipped cream! Perfect as a last-minute dessert for a gathering because it’s delicious, simple, and highlights seasonal produce.