Preheat the oven to 350ºF and lightly grease a 9X13 (23 X 33 cm) baking dish. Set aside
Add the strawberries, rhubarb, vanilla extract, sugar, and 2 tablespoons of flour (or 1 tablespoon of cornstarch) into a large bowl. Mix well to combine.
Transfer the strawberry rhubarb fruit filling to the prepared baking dish, spreading it into an even layer.
Combine the flour, rolled oats, brown sugar, cinnamon, baking powder, and baking soda in a separate large bowl. Whisk well.
Add the melted butter to the oatmeal mixture and stir well to coat everything well. Alternatively, If you prefer a crunchier topping, use cold cubed butter and cut it in until the mixture resembles coarse crumbs.
Transfer the oatmeal topping to the baking dish, spreading it in an even layer over the fruit filling.
Bake for 40-45 minutes, or until bubbly and browned.
Remove from the oven and cool for 10 minutes before serving.
Scoop into bowls and serve with a scoop of vanilla ice cream or a dollop of whipped cream.