Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, combine the strawberries, rhubarb, 1/2 cup granulated sugar, cornstarch, vanilla extract, and lemon juice. Toss until the fruit is well coated. Pour the mixture into the prepared baking dish.
In another bowl, mix together the rolled oats, flour, brown sugar, cinnamon, and the 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 filling in the baking dish.
Bake in the preheated oven for 35-40 minutes, or until the topping is golden brown and the fruit is bubbling.
Remove from the oven and let cool for at least 10 minutes before serving.