Mix rhubarb and sugar together and add to a 9x13 baking dish. Sprinkle 3 Tbsp of flour on top.
In a bowl, mix together brown sugar, oatmeal, and flour. Cut in cold shredded butter. Mix well until it is evenly crumbly and well combined. Spread out the topping evenly over the rhubarb.
Bake in preheated oven for 45 minutes or until the top is golden brown and rhubarb is bubbly.
Notes
This can be frozen prior to baking. Use a freezer-safe baking dish and cover tightly with foil. Seal in a 2-gallon size freezer-safe bag, removing any extra air. Bake from frozen, adding 15 minutes or so to the time.