Preheat oven to 350°F and lay your pie crust in a 9-inch pie pan. Chill while you prepare the filling.
1 pie crust
If not already done, chop rhubarb into small pieces, about the size of blueberries.
10 ounces rhubarb
In a bowl, mix together 1 cup sugar and 1/3 cup flour. In another bowl, whisk together eggs, sour cream, and vanilla until blended, then stir into sugar-flour mixture and whisk until smooth.
1 cup granulated sugar, 1/3 cup all-purpose flour, 2 large eggs, 1/2 cup sour cream, 1 teaspoon pure vanilla extract
For the topping, mix together 1/2 cup flour, brown sugar, 2 tablespoons granulated sugar, cinnamon, and melted butter with a fork until it is blended and crumbly. (You can also do this in a food processor using the pulse function.)
1/2 cup all-purpose flour, 4 tablespoons melted butter, 2 tablespoons brown sugar, 2 tablespoons granulated sugar, 1/2 teaspoon ground cinnamon
Place chilled crust on a baking sheet. Add the rhubarb in an even layer. Pour the custard mixture over the rhubarb; it's OK if it isn't completely covered. Scatter on the crumble topping.
Bake for about 50 minutes, until top is golden. Let cool completely before cutting.
Notes
You can make this recipe gluten free by using a gluten free pie crust and swapping the flour for gluten free flour or almond meal.
If using frozen rhubarb, make sure it is completely thawed and well drained.