Add the sliced rhubarb and strawberries into a large bowl along with the granulated sugar, cornstarch, and lemon juice. Tip: if using frozen rhubarb, thaw and drain beforehand.
Gently mix to combine everything evenly and set aside.
Use a pastry cutter or blender to mix and cut the butter into the dry ingredients. Mix until the butter is the size of a pea.
Add the strawberry rhubarb mixture to a 9x13-inch baking dish and spread it out evenly.
Then evenly sprinkle on the crisp topping. It may look like a lot, but trust me it won’t be too much once it’s baked.
Preheat your oven to 375 degrees F and bake for about 30 minutes or until it’s bubbling and the topping is golden brown. Serve warm with vanilla ice cream.