Place the cold mashed potatoes in a large bowl. Add the egg, Parmesan, flour, garlic, chopped herbs, salt, and black pepper, then mix just until the ingredients are evenly combined.

Check the consistency before shaping. If the mixture is too soft or sticky to hold a patty shape, add breadcrumbs or another tablespoon of flour, using as little extra binder as possible so the cakes remain tender.

Divide the mixture into 8 portions and shape each portion into a patty about 1/2 inch thick. For a more pronounced crust, lightly coat both sides of every patty with plain breadcrumbs.

If the cakes feel delicate, place them in the refrigerator for 15 to 30 minutes. This optional chilling step helps them hold together and allows the outside to brown before the potato centers become too soft.

Heat 2 to 3 tablespoons of oil in a large nonstick or cast-iron skillet over medium heat. Fry the cakes in batches, leaving space between them, for approximately 3 to 4 minutes per side, until deep golden brown and crisp.

Transfer the finished potato cakes to a paper-towel-lined plate or wire rack. Serve hot with sour cream, crème fraîche, garlic aioli, chopped herbs, or your favorite egg and vegetable sides.