- Preheat the oven to 325°F. Grease an 8-inch (or other 6-cup volume) baking dish.
- For the topping, combine flour, oats, sugar, cinnamon, salt and butter in a bowl. Work together with fingertips until crumbly.
- Toss the rhubarb with the brown sugar and vanilla. Spoon this into the prepared baking dish. Sprinkle the topping evenly over the fruit. Bake the crisp until it is bubbly and the top is evenly browned, about 30 minutes. Let cool slightly. Serve warm, topped with ice cream or cream, if desired.