Step 1

Preheat oven to 375 degrees. Butter six 8-ounce ramekins (or a 2 1/2-quart shallow casserole), and set aside. Make crumb mixture: In a large bowl, whisk together flour, brown sugar, cinnamon, nutmeg, salt, and oats. Using a pastry cutter or two knives, cut in chilled butter until mixture comes together into clumps, about 4 to 5 minutes; set aside.

Step 2

In a large bowl, stir to combine rhubarb, zest, orange juice, and vanilla seeds. In a small bowl, combine sugar and cornstarch; sprinkle over rhubarb until just coated. Pour 1 cup rhubarb mixture into each ramekin, and top with 1/2 cup crumb mixture. Bake until the rhubarb is tender when pierced with the tip of a knife, about 30 minutes (or about 40 minutes for the casserole). Transfer from oven to a cooling rack. Let cool slightly, and serve warm.