Preheat the oven to 350F. To make the filling, toss the rhubarb, strawberries, sugar and cornstarch in a bowl or directly in your baking dish; spread it out in the dish.

To make the crumble, blend all the ingredients in a bowl with a fork or your fingers, or pulse it in the food processor until well combined and crumbly. Sprinkle it over the fruit, squeezing to create larger clumps as you go.

Bake for 45 minutes to an hour, until golden and bubbly around the edges.