Set your oven to 350°F (175°C).
In a medium bowl, combine the flour, oats, brown sugar, granulated sugar, and salt. Add the softened butter and use your fingers to mix until the mixture resembles coarse crumbs. Set aside.
In another medium bowl, mix the sliced strawberries, chopped rhubarb, sugar, cornstarch, lemon juice, vanilla extract, and salt until well combined.
Transfer the fruit filling to an ungreased 8-inch baking dish. Evenly distribute the crumble topping over the fruit, breaking up any large clumps.
Bake: Place the dish on the middle rack of the oven and bake for 55 to 60 minutes, or until the topping is golden brown and the filling is bubbling.
Remove the crisp from the oven and let it cool on a wire rack for at least 1 hour. This allows the flavours to meld and the filling to set. Serve warm, ideally with a scoop of vanilla ice cream.