This rhubarb crisp recipe is one you'll reach for again and again. The buttery oat and cinnamon topping is simply irresistible. This rhubarb crisp is unique because it is assembled with a crumb bottom and top crust, giving you a double dose of the delicious crumble mixture. If you prefer only the topping, feel free to cut the crumble ingredients by half and omit the bottom crust, or bake it with a doubled topping. Or you might like a version made with a flour crumble topping. Rhubarb has long been a popular choice for pies, cakes, preserves, and compotes or sauces. Its nickname is "Pie plant. " All rhubarbwhether red or greenis tart, but some varieties might be less tart or a bit sweeter than others. Feel free to cut the granulated sugar back to 3/4 cup if you want to taste the tartness of the rhubarb.