Preheat the oven to 375°F (190°C). In a mixing bowl, combine the chopped rhubarb, granulated sugar, and vanilla extract. Toss until the rhubarb is evenly coated and set aside to macerate for about 10 minutes.
4 cups rhubarb stalks, 1 cup granulated sugar, 1 teaspoon vanilla extract
Spread the rhubarb mixture evenly into a 9x13 inch baking dish, smoothing the surface with the back of a spoon.
4 cups rhubarb stalks
Prepare the crumble topping by combining oats, flour, brown sugar, and salt in a bowl. Add the cold, cubed butter and use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs with small pea-sized bits.
1 cup old-fashioned rolled oats, 1 cup all-purpose flour, 1/2 cup brown sugar, 1/2 cup unsalted butter, 1/4 teaspoon salt
Spread the crumble evenly over the rhubarb filling, covering it completely and patting gently to adhere.
1 cup old-fashioned rolled oats, 1 cup all-purpose flour, 1/2 cup brown sugar, 1/2 cup unsalted butter, 1/4 teaspoon salt
Bake in the preheated oven for 40-45 minutes, or until the topping is golden brown, and the filling is bubbling around the edges. You should hear gentle bubbling and see a rich, caramelized crust on top.
Remove from the oven and let cool slightly before serving. The crisp should be hot and aromatic, with a crunchy topping contrasting the tender, tart filling.
