Wash and slice the strawberries and rhubarb evenly. Mix them with sugar and flour until all the fruit is coated. This helps create a thick, juicy filling instead of a watery one.
In a separate bowl, combine flour, brown sugar, oats, and cold butter. Cut the butter into the dry ingredients until the mixture resembles coarse crumbs. This step is key to a crispy topping.
Spread the fruit mixture evenly into your baking dish. Sprinkle the crumble topping over the fruit without pressing it down. This keeps the topping light and crunchy.
Bake in a preheated oven until the topping turns golden brown and the fruit mixture bubbles around the edges. The smell alone will tell you it’s almost ready.
Let the crisp cool slightly before serving. This allows the filling to set and makes serving easier.