Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the sliced strawberries, chopped rhubarb, granulated sugar, cornstarch, vanilla extract, and lemon juice. Mix until well combined, and pour the mixture into the prepared 9x13 inch baking dish.
In another bowl, mix together rolled oats, all-purpose flour, brown sugar, granulated sugar, and ground cinnamon. Add the melted butter and stir until the mixture resembles coarse crumbs.
Sprinkle the oat topping evenly over the fruit filling in the baking dish.
Bake in the preheated oven for about 40 minutes, or until the topping is golden brown and the fruit is bubbling.
Remove from the oven and let cool slightly before serving.
Serve warm with a scoop of vanilla ice cream or whipped cream for an extra treat.