Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish (or equivalent 2.5-quart dish) with butter or cooking spray.
Step 2: In a large bowl, gently combine the sliced rhubarb, hulled and quartered strawberries, 3/4 cup of the granulated sugar, cornstarch, and vanilla extract. Toss until the fruit is evenly coated. Set aside.
Step 3: In a separate medium bowl, whisk together the all-purpose flour, old-fashioned oats, light brown sugar, remaining 1/4 cup granulated sugar, ground cinnamon, and salt.
Step 4: Add the cold, cubed unsalted butter to the dry ingredients. Using your fingertips, a pastry blender, or two knives, cut the butter into the mixture until it resembles coarse crumbs with some pea-sized pieces remaining.
Step 5: Pour the fruit mixture into the prepared baking dish, spreading it into an even layer.
Step 6: Evenly sprinkle the crumble topping over the fruit layer, ensuring it is mostly covered.
Step 7: Bake for 40-50 minutes, or until the topping is golden brown and the fruit filling is bubbly and tender when pierced with a fork. If the topping browns too quickly, loosely tent the dish with foil.
Step 8: Remove from the oven and let the crumble cool on a wire rack for at least 15-20 minutes before serving. This allows the fruit filling to set properly.