Step 1: In a large mixing bowl, combine the cold mashed potatoes, shredded cheddar cheese, beaten egg, all-purpose flour, chopped green onions, and garlic powder. Mix until well combined but do not overwork.
Step 2: Season the mixture with salt and pepper to taste. Note that the fried mashed potato patties recipe outcome depends on how well-seasoned your original mashed potatoes were.
Step 3: Scoop about 1/4 cup of the potato mixture and shape it into a ball, then flatten it into a patty about 1/2 inch thick. Repeat until all the mixture is used.
Step 4: Heat the vegetable oil in a large non-stick skillet over medium-high heat. Carefully place the patties in the hot oil, working in batches to avoid crowding the pan.
Step 5: Fry the patties for 3-4 minutes per side until they are golden brown and crispy. Remove from the pan and serve this fried mashed potato patties recipe immediately while hot.