• Preheat the oven to 400°F, and lightly grease an 8-inch round, 8 x 8-inch baking dish, or 2-quart dish.

For the Crumb Topping

• In a medium bowl, use a pastry cutter or your hands to combine the topping ingredients until well mixed.
  1 cup old-fashioned rolled oats, 6 tablespoons unsalted butter, ½ cup all-purpose flour, ½ cup brown sugar, ¼ cup granulated sugar, ½ teaspoon ground cinnamon, ½ teaspoon salt
• Transfer the crumb topping to the refrigerator while you prepare the filling.

For the Rhubarb Filling

• Add the rhubarb, brown sugar, ¼ cup of water, vanilla and cinnamon to a medium saucepan. Bring to a simmer over medium heat, and cook for about 5 minutes.
  6 cups rhubarb, ¾ cup light brown sugar, 2 teaspoons pure vanilla extract, 1 teaspoon ground cinnamon
• While the rhubarb mixture cooks, make a slurry by mixing the remaining ¼ cup cold water and 2 tablespoons of cornstarch until smooth. Add the slurry, and stir until the mixture beings to thicken, about 2 to 3 minutes.
  2 tablespoons cornstarch
• Add the rhubarb mixture to the prepared baking dish then sprinkle the crumb topping over the top.
• Bake for 30 to 35 minutes or until the top turns a light golden color and the center jiggles slightly. The filling will be bubbly around the edges.
• Allow the crisp to rest for 5-10 minutes before serving. Serve warm with vanilla ice cream!