In a large mixing bowl, combine the leftover mashed potatoes, 2 eggs, garlic powder, 1/4 cup flour, chopped green onions, and grated cheese. Mix until well combined. The mixture should hold together but not be too dry. If the mixture is too wet, add more flour 1 tablespoon at a time.
Options for forming the potato cakes - If using egg molds, fill them with the potato mixture and smooth the tops. Place in the freezer for about 30 minutes to firm up. If not using molds, you can form the mixture into patties by hand or skip this step and spoon the potato mixture directly on the griddle or in the skillet.
Heat the griddle on low heat (about 350°F for griddle cooking). *If cooking mashed potato cakes in a skillet, heat a large skillet over medium-low heat. Add enough bacon grease or cooking oil to the griddle or skillet to shallow fry.
Optional Step - If using the panko, then sprinkle a thin even layer of panko breadcrumbs where you’ll place each potato cake. Spoon the potato mixture directly on top of the panko, and press down into a round "pancake" shape. Sprinkle another thin layer of panko on top of each potato cake.
If not using panko, spoon the potato mixture directly in the oil, about ½ cup to ⅔ cup at a time, and use your spatula to gently press down into a "pancake" shape.
Cook for 5-6 minutes until the bottoms of the potato cakes are golden and crispy. Flip carefully and cook for another 4-5 minutes until the second side is crispy and the cakes are heated through.
Remove potato cakes from the griddle and serve immediately with your favorite toppings.