Ingredients
1 cup all-purpose flour
3/4 cup brown sugar
¾ cup oats
½ cup melted butter
1 teaspoon ground cinnamon
4 cups diced rhubarb
3/4 cup white sugar
1 cup water
2 tablespoons cornstarch
1 teaspoon vanilla extract
Directions
Preheat the oven to 350 degrees F (175 degrees C). Grease a 9×13-inch baking dish.
Mix flour, brown sugar, oats, butter, and cinnamon together in a bowl until crumbly.
Put rhubarb in bottom of baking dish.
Combine white sugar, water, cornstarch, and vanilla in a saucepan over medium heat; cook and stir until thick and clear, about 5 minutes. Pour sugar mixture over rhubarb. Sprinkle oat mixture over rhubarb.
Bake in the preheated oven until the crisp is lightly brown and bubbling, about 1 hour.
borrowed from: https://www.allrecipes.com/recipe/242191/easy-rhubarb-crisp/