Mix the filling: In a large bowl, combine the rhubarb, 3 tablespoons of flour, and 1 1/2 cups of sugar. Toss until the rhubarb is evenly coated, then pour this mixture into the bottom of a 9 x 13 pan.
Prepare the topping: In another bowl, mix the brown sugar, 1 1/2 cups of flour, and oatmeal together. Cut in the butter using a pastry blender or your fingers until the mixture resembles coarse oats.
Layer the topping: Sprinkle the topping evenly over the rhubarb filling, gently patting it down to create an even layer.
Bake: Preheat your oven to 350°F (175°C) and bake the crisp for about 45 minutes until bubbling and golden brown.
Serve: Enjoy your rhubarb crisp warm or cold, either plain or with ice cream.
Notes
For an extra indulgent touch, drizzle with caramel sauce.