Preheat the Oven: Set it to 350°F (175°C). Try not to forget this step like I almost did.
Make the Filling: In a bowl, mix brown sugar, white sugar, oats, flour, and cinnamon.
Add Fruit: Toss in rhubarb and blueberries, mix until coated, then pour into a baking dish.
Prepare the Topping: In another bowl, mix flour, oats, cinnamon, and nutmeg.
Add Butter: Pour in about half the butter and mix until crumbly.
Assemble: Sprinkle the topping over the fruit mixture.
Finish with Butter: Drizzle the remaining butter over the top like you’re making a bold decision.
Bake: Place in the oven for 20–30 minutes until golden and bubbly.
Cool Slightly: Let it sit for about 5 minutes before digging in.