Preheat and Prep: Preheat your oven to 350°F (175°C). Lightly grease a standard 9x9 inch square baking dish with butter or non-stick cooking spray.

Make the Filling: In a large mixing bowl, combine the chopped rhubarb and quartered strawberries. Sprinkle the granulated sugar and cornstarch evenly over the fruit. Add the vanilla extract and fresh lemon juice. Toss everything together gently until the fruit is evenly coated and the dry ingredients have dissolved into a light syrup.

Transfer to Dish: Pour the fruit filling directly into your prepared baking dish. Use a spatula to spread it out into an even layer. Set aside.

Make the Topping: In a separate medium mixing bowl, whisk together the old-fashioned oats, all-purpose flour, brown sugar, ground cinnamon, and salt.

Cut in the Butter: Add the cold, cubed butter to the dry oat mixture. Using a pastry blender or your fingers, quickly cut and rub the butter into the dry ingredients until the mixture resembles coarse, wet sand and clumps together when you squeeze it in your hand.

Bake: Sprinkle the crumble topping evenly over the fruit filling in the baking dish. Bake in the preheated oven for 40 to 45 minutes, or until the topping is deep golden brown and the fruit juices are bubbling rapidly around the edges of the pan.

Cool and Serve: Remove the crisp from the oven and let it cool on a wire rack for at least 15 to 20 minutes before serving. This allows the hot fruit juices to thicken up. Serve warm with a generous scoop of vanilla ice cream or fresh whipped cream.