Prep the oven: Preheat your oven to 375°F (190°C). Grease your 9×13-inch baking dish lightly.
Make the rhubarb layer: In a mixing bowl, toss together the diced rhubarb, white sugar, and 3 tablespoons of flour. Spread this evenly in the baking dish.
Make the crumble topping: In another bowl, mix flour, brown sugar, and oats. Add in the cold butter and work it in with your fingers or a pastry cutter until the mixture resembles coarse crumbs.
Assemble: Sprinkle the crumble mixture evenly over the rhubarb layer.
Bake: Place the dish in the oven and bake for 35–40 minutes or until the top is golden brown and the rhubarb is bubbling around the edges.
Cool slightly before serving: Let it sit for 10 minutes before digging in so the filling can thicken a bit.