Prep your skillet: Set a large nonstick or cast-iron skillet over medium heat. Add 1 tablespoon of oil or butter. You’ll use the rest later for crisping the top and sides.

Sauté the onion and corn: Add the chopped onion and cook for 3–4 minutes until softened. Stir in the corn and cook another 2–3 minutes. If using garlic, add it for the last 30 seconds. Season lightly with salt and pepper, then transfer the mixture to a bowl to cool slightly.

Mix the potato base: In a large bowl, combine the leftover mashed potatoes, eggs, garlic powder (if not using fresh garlic), and a pinch of paprika or chili flakes. Fold in the onion-corn mixture and green onions. If using cheese, stir in half and reserve the rest for topping. Taste and adjust salt and pepper.

Heat and spread: Return the skillet to medium heat with the remaining oil or butter. Spoon in the potato mixture and spread it evenly, pressing gently with a spatula to form a firm, even layer about 1 to 1.5 inches thick.

Cook until the bottom crisps: Let it cook undisturbed for 6–8 minutes. You want a golden-brown crust on the bottom. Peek at the edges with a spatula. If it’s browning too fast, lower the heat.

Finish the top: Sprinkle the remaining cheese over the surface. Cover with a lid and cook another 6–8 minutes on low heat until the center is set and the cheese melts. If you don’t have a lid, tent with foil or flip the potato cake carefully in sections to crisp both sides. It doesn’t have to be perfect—rustic is fine.

Rest and slice: Turn off the heat and let it sit for 3–5 minutes. This helps it firm up so it slices cleanly. Cut into wedges and serve warm.