Preheat oven to 350 degrees F. Spray or grease your cast iron skill or pie pan. Set aside. Combine 2 cups flour, 1 cup melted butter, and 1/2 cup powdered sugar plus an addition 2 Tbsp. Spread the dough evenly in the pan, going up the sides if you can. Bake for 15 minutes or until the crust is starting to brown on the edges and feels a little crisp in the center.
While the crust is cooking, start to make the custard filling.
Custard Filling
Add 1 cup of sugar to the cut rhubarb and stir. This will help sweeten the rhubarb and keep it moist. In a large mixing bowl beat the 3 eggs. Add 3 Tbsp heavy whipping cream or milk, 1 cup white sugar, 1/4 cup flour, and 1 tsp. nutmeg. Mix well. Add the 4 cups of cut rhubarb and sugar to the mixture. Stir until combined. Set aside.
Crumb Topping
Combine 1 cup flour, 1/2 cup melted butter, and 1/2 cup brown sugar. Mix together to make a nice crumble.
Rhubarb Pie Instructions
Pour the custard mixture on top of the crust. Drop 1 Tbsp cubbed butter across the top. The last step is to sprinkle the crumb mixture to the top.
Bake for approximately 50-60 minutes. The pie is done when a toothpick comes out clean in the center of the pie. In a 9-inch cast-iron skillet, this pie took me an hour and 10 minutes to cook. It seemed moist in the center, but the toothpick came out clean. The middle did form more as it sat in the pan.