Heat oven to 375°F. Lightly butter an 8-inch square baking dish.
Mix rhubarb, granulated sugar, cornstarch, flour, lemon juice, and vanilla in a large bowl until coated.
Spread the rhubarb mixture into the prepared dish and let it sit while you make the topping.
Stir oats, brown sugar, flour, cinnamon, and salt in a bowl. Drizzle in melted butter and toss until clumpy.
Scatter the oat mixture evenly over the rhubarb, covering to the edges.
Bake until the topping is deep golden and the fruit is bubbling thickly, 35 to 40 minutes.
Cool 15 minutes to set the juices. Serve warm, ideally with vanilla ice cream or lightly sweetened whipped cream.
Notes
Variation: Swap in 1 cup sliced strawberries for part of the rhubarb for a sweeter twist, or add 1/2 cup chopped toasted almonds to the topping for crunch.Storage: Refrigerate leftovers covered for up to 3 days; rewarm in a 325°F oven until crisp again. The unbaked assembled crisp can be refrigerated for 6 hours before baking.This recipe is an original creation inspired by classic Rhubarb Crisp Perfection flavors. All ingredient ratios and instructions are independently developed.