Preheat your oven to 350°F.
In a large mixing bowl, combine the strawberries, rhubarb, 1/2 cup of granulated sugar, cornstarch, vanilla extract, and lemon juice. Stir until the fruit is well coated.
Pour the fruit mixture into a greased 9x13-inch baking dish.
In another bowl, mix the rolled oats, flour, brown sugar, cinnamon, and remaining 1/2 cup of granulated sugar.
Pour in the melted butter and stir until the mixture is crumbly.
Sprinkle the oat mixture evenly over the fruit.
Bake in the preheated oven for 35-40 minutes, or until the top is golden brown and the fruit is bubbling.
Allow to cool for at least 10 minutes before serving.