Directions

1. Preheat oven to 350°F with a rack set in the center.
2. Trim and discard ends and any leaves from rhubarb. If any stalks are really large, cut them in half lengthwise. Slice stalks crosswise about 1/2-inch wide to make 6 cups.
3. In a large bowl, combine rhubarb, granulated sugar, 1/2 cup brown sugar, the cornstarch, lemon zest and juice, and vanilla, using a rubber spatula to toss and coat ingredients. Arrange filling evenly in an 8-inch square baking dish or other shallow 1 1/2-qt. baking dish.
4. Put remaining 3/4 cup brown sugar, the flour, oats, butter, cinnamon, and salt in the bowl of a food processor. Pulse until butter is evenly distributed and mixture is a little clumpy. Sprinkle streusel over rhubarb mixture.
5. Bake crisp until filling is bubbly and streusel is evenly browned, 45-50 minutes. Let cool on a rack about 15 minutes. Serve the crisp warm, with scoops of ice cream if you like.