1. Prepare Oven and Baking Dish:

• Preheat your oven to 350°F (175°C).
• Grease an 8×8 inch baking dish. Set aside.

2. Make the Rhubarb Filling:

• Prepare your rhubarb: If using fresh, wash, trim the ends, and chop the stalks into roughly 1-inch pieces. If using frozen, you can use it directly.
• In a large mixing bowl, combine the 4 cups of chopped rhubarb, 1 cup granulated sugar, 1/4 cup all-purpose flour, 1 tablespoon lemon juice, and 1 teaspoon vanilla extract.
• Stir gently with a spatula or spoon until the rhubarb pieces are evenly coated with the sugar and flour mixture.
• Transfer this rhubarb filling mixture into the prepared greased 8×8 inch baking dish and spread it into an even layer.  

3. Make the Crisp Topping:

• In a separate medium bowl, combine the 1 cup rolled oats, 1/2 cup all-purpose flour, 1/2 cup packed brown sugar, and 1/2 teaspoon ground cinnamon. Whisk or stir to mix the dry ingredients.
• Pour the 1/2 cup of melted unsalted butter over the dry oat mixture.
• Stir with a fork or spatula until the mixture is well combined and forms coarse crumbles. All the dry ingredients should be moistened by the butter.

4. Assemble and Bake:

• Sprinkle the prepared oat crumble mixture evenly over the top of the rhubarb filling in the baking dish, ensuring full coverage.
• Place the baking dish in the preheated oven.
• Bake for 40-50 minutes, or until the rhubarb filling is bubbly around the edges and the crumble topping is golden brown and looks crisp.

5. Cool Slightly and Serve:

• Carefully remove the hot Rhubarb Crisp from the oven.
• Place the baking dish on a wire rack. Allow the crisp to cool for at least 10-15 minutes before serving. This allows the bubbly fruit filling to thicken slightly as it cools.
• Serve warm, perhaps with a scoop of vanilla ice cream or a dollop of whipped cream. Enjoy!