Step 1: Preheat your oven to 375°F (190°C).
Step 2: In a large bowl, combine the chopped rhubarb with 1/2 cup granulated sugar, 1/4 cup all-purpose flour, and 1 teaspoon vanilla extract. Toss gently until the rhubarb is evenly coated.
Step 3: Pour the rhubarb mixture into an 8x8 inch (or similar 2-quart) baking dish. Spread it out evenly.
Step 4: In a separate medium bowl, melt 1/2 cup (1 stick) unsalted butter.
Step 5: Add 1 cup old-fashioned rolled oats, 1/2 cup packed light brown sugar, 1/2 cup all-purpose flour, 1/2 teaspoon ground cinnamon, and 1/4 teaspoon salt to the bowl with the melted butter.
Step 6: Use a fork or your fingers to mix everything together until well combined and a crumbly topping forms.
Step 7: Evenly sprinkle the oat topping over the rhubarb filling in the baking dish.
Step 8: Bake for 35-45 minutes, or until the topping is golden brown and the rhubarb filling is bubbly and tender when pierced with a fork. If the topping starts to brown too quickly, you can loosely tent the dish with foil.
Step 9: Let the crisp cool for at least 15-20 minutes before serving. This allows the filling to set slightly. Serve warm with a dollop of vanilla ice cream or fresh whipped cream.