1. In a large bowl, stir together the chopped onion, mashed potatoes, salt, and pepper. Measure the flour onto a shallow plate. 2. With your hands, mold the mixture into 6 patties (using about 1/2 cup for each). Dredge the patties in flour and set aside. 3. In a large skillet, melt the butter over medium heat. Cook the potato cakes in the melted butter, about 2 minutes per side, flipping once. When cakes are golden brown on the outside and heated through, remove from the pan and keep warm until serving time. 4. You may add any vegetable puree you have into the middle of the pattie.