Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the sliced strawberries, chopped rhubarb, granulated sugar, cornstarch, and vanilla extract. Stir gently until the fruit is coated. Transfer this mixture to the greased 9x13 inch baking dish.
In another bowl, mix the rolled oats, flour, brown sugar, and ground cinnamon. Pour in the melted butter and stir until the mixture is crumbly and well combined.
Spread the oatmeal mixture evenly over the fruit filling in the baking dish.
Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and the fruit is bubbly.
Remove from the oven and let it cool for at least 10 minutes before serving.
Serve warm or at room temperature, possibly with a scoop of vanilla ice cream or whipped cream for added indulgence.