If your mashed potatoes are warm, spread them on a plate and let cool to room temperature, or pop them in the fridge for 10 minutes. Cooler potatoes hold together better.
In a large bowl, combine the mashed potatoes, egg, grated cheese if using, chopped herbs, garlic powder, salt, and pepper. Mix until everything is evenly distributed. Don t overmix; stop once it s cohesive.
Add 1/4 cup of the flour to the potato mixture and gently fold it in. If the mixture feels too sticky, add the remaining flour a tablespoon at a time until you can form patties that hold their shape.
Dust a clean work surface with a little flour. Scoop about 1/3 cup of potato mixture for each patty and shape into rounds about 3 inches across and 1/2 inch thick. Lightly dust each patty with flour.
Heat the oil in a large nonstick or cast iron skillet over medium heat. It s ready when a small pinch of flour sizzles. Don t let the oil smoke.
Place the patties in the skillet in a single layer, leaving space between them. Fry for 3 to 4 minutes on the first side, until golden brown, then carefully flip and fry another 3 minutes. Lower the heat slightly if they brown too fast.
Transfer cooked patties to a plate lined with paper towels to drain briefly. Season with a pinch more salt while hot, if desired.
Serve warm with a dollop of sour cream, a sprinkling of extra herbs, ketchup, or your favorite dipping sauce.
Leftovers can be kept in an airtight container in the refrigerator for up to 3 days. Reheat in a skillet over medium heat for best texture.