01 - Preheat oven to 350°F (175°C). Lightly grease an 8-inch square baking dish.
02 - In a large mixing bowl, toss the hulled and quartered strawberries, sliced rhubarb, granulated sugar, cornstarch, vanilla extract, and lemon juice together until the fruit is evenly coated.
03 - Spread the fruit mixture in an even layer across the bottom of the prepared baking dish.
04 - In a separate bowl, combine the rolled oats, all-purpose flour, packed brown sugar, cinnamon, and salt. Add the cold cubed butter and use your fingers or a pastry cutter to cut it into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
05 - Sprinkle the crumb topping evenly over the fruit filling, covering the entire surface.
06 - Bake for 35 to 40 minutes, until the topping is golden brown and the fruit filling is bubbling around the edges.
07 - Let the crisp cool for 10 to 15 minutes before serving. Serve warm, optionally with a scoop of vanilla ice cream or a dollop of whipped cream.