In a large bowl, combine the leftover mashed potatoes, shredded cheddar cheese, crumbled bacon, and chopped green onions. Mix until well combined.
Add the flour, egg, garlic powder, salt, and black pepper to the potato mixture. Stir until everything is evenly incorporated.
Form the mixture into patties, about 2-3 inches in diameter and 1/2 inch thick. You should get about 8-10 patties.
Heat the vegetable oil in a large skillet over medium heat.
Once the oil is hot, carefully place the potato cakes in the skillet, making sure not to overcrowd the pan. Fry for about 4-5 minutes on each side, or until golden brown and crispy.
Remove the cakes from the skillet and place them on a paper towel-lined plate to drain excess oil.
Serve warm, garnished with additional green onions if desired.