HEat oven to 375 degrees, Use butter paper to butter six 8-ounce ramekins or a 2 1/2 quart shallow casserole and set aside. In a food prosessor, place the flour, brown sugar, cinnamon, nutmeg, salt, and oats. Pulse ingredients twice. Add butter and pulse until mixture comes together in clumps; set aside.
In a large bowl, combine rhubarb, juice, zest and vanilla. In a small bowl combine sugar and cornstarch, then combine with rhubarb mixture. Stir. Pour 1 cup rhubarb mixture into each ramekin, and top with 1/2 cup crumb mixture. Bake for 30 minutes for ramekins and 40 for casserole. Transfer from oven to a cooling rack. Let cool slightly before serving.