Preheat the oven to 350 degrees Fahrenheit and spray an 8 x 8 x 2-inch, or 9 x 9 x 2-inch baking dish with nonstick cooking spray.
Add 4 cups trimmed and halved or quartered strawberries , 2 cups chopped fresh rhubarb , ½ cup granulated sugar, 2 tablespoons cornstarch, 1 teaspoon vanilla extract, and 2 tablespoons fresh lemon juice to a large bowl and stir until the fruit is evenly coated. Transfer into the prepared baking dish.
In a separate bowl, combine 1 cup rolled oats, ⅓ cup all-purpose flour, ⅓ cup packed brown sugar, and ¼ teaspoon salt. Add ½ cup softened unsalted butter and use clean hands to rub the butter into the oat mixture until combined and crumbly. Sprinkle the crumble evenly over the top of the strawberry mixture.
Bake 45-50 minutes, until the strawberry mixture is thick and bubbling around the edges and the crumble topping is golden brown.
Serve warm with a scoop of vanilla ice cream or whipped cream, if desired.