Preheat oven to 350°F (175°C). Lightly grease an 8x8-inch baking dish.
Make the filling on the stove: In a medium saucepan, combine strawberries, rhubarb, sugar, cornstarch, and vanilla extract all at once.Cook over medium heat, stirring often, until the fruit softens and the mixture thickens, about 8–10 minutes.
If the filling gets too thick or sticky while cooking, add 1–2 tablespoons of water as needed to loosen it to your desired consistency.
Once thickened but pourable, remove from heat and pour into the greased baking dish.
Prepare the crumble topping: In a bowl, mix together the flour, oats, and brown sugar.Add the melted butter and stir until the mixture forms coarse crumbs.
Assemble and bake: Sprinkle the crumble mixture evenly over the warm filling.
Bake for 30–35 minutes, or until the top is golden and the fruit is bubbling at the edges.