Preheat oven to 375°F (190°C). Lightly grease your baking dish.
In a large bowl, combine the strawberries, rhubarb, sugar, cornstarch, and vanilla extract (if using). Toss to coat everything evenly.
Transfer the fruit mixture to the prepared baking dish.
In a separate bowl, combine the oats, flour, brown sugar, cinnamon, and salt. Using a pastry cutter or your fingers, work in the cold butter until the mixture resembles coarse crumbs.
Sprinkle the crumb topping evenly over the fruit filling.
Bake for 40-45 minutes, or until the topping is golden brown and the fruit is bubbly.
Let cool slightly before serving.