01 - Preheat oven to 350°F. Lightly grease an 8-inch square baking dish and set aside.
02 - In a large mixing bowl, combine the strawberries, rhubarb, granulated sugar, cornstarch, vanilla extract, and lemon juice. Toss gently until all the fruit is evenly coated and the sugar and cornstarch are well distributed.
03 - Spread the fruit mixture evenly into the prepared baking dish, ensuring an even layer across the bottom.
04 - In a separate bowl, whisk together the oats, flour, 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 topping evenly over the fruit filling, covering the surface completely.
06 - Bake for 35 to 40 minutes until the topping is golden brown and the fruit filling is bubbly around the edges.
07 - Allow the crisp to cool for 10 to 15 minutes before serving. Serve warm with vanilla ice cream or whipped cream if desired.