Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking dish or a similar-sized casserole dish.
In a large mixing bowl, combine the sliced strawberries, chopped rhubarb, 1/2 cup of granulated sugar, cornstarch, vanilla extract, and lemon juice. Toss until the fruit is well coated. Pour this mixture into the prepared baking dish.
In another bowl, mix together the rolled oats, flour, brown sugar, cinnamon, and salt. Drizzle the melted butter over the oat mixture and stir until crumbly.
Evenly sprinkle the oat topping over the fruit mixture 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 it cool for about 10 minutes before serving.