In a large mixing bowl, combine the diced rhubarb, peeled and diced apples, sugar, and cornstarch. Gently mix until well-coated. Set aside.

Preheat your oven to 350°F (175°C) and grease an 8x8-inch baking pan.

Pour the prepared fruit mixture into the greased baking pan, spreading it evenly across the bottom.

In a separate bowl, melt butter and combine it with oats, brown sugar, flour, and cinnamon. Stir until well mixed.

Evenly sprinkle the oat mixture over the fruit filling.

Bake for 40-45 minutes until the topping turns golden brown and edges bubble.

Remove from oven and let the crumble cool on a wire rack for at least 15 minutes before serving.