Preheat oven to 350°F (175°C). Grease or line a 9x13-inch baking dish.
Mix flour, oats, brown sugar, and melted butter in a large bowl. Stir in nuts if using. Set aside 1 1/2 cups for topping.
Press remaining mixture into the dish. Bake for 10 minutes. Cool slightly.
Beat cream cheese, sugar, egg, and vanilla until smooth. Spread over crust.
In a saucepan, combine rhubarb, sugar, cornstarch, and water. Cook over medium heat for 8–10 minutes, until thickened. Cool slightly.
Spread rhubarb filling over cream cheese layer.
Sprinkle reserved crumble on top.
Bake 30–35 minutes, until golden and set.
Cool completely, then chill for at least 2 hours before slicing.