Begin by placing a large, heavy-bottom skillet over medium-high heat. Toss in the chopped bacon or pancetta. Cook until the pieces become nice and crispy, which should take about 10 minutes. Stir occasionally to ensure even cooking. Once done, carefully remove the bacon, draining the excess fat on a paper towel-lined plate. Keep just about 1 tablespoon of the rendered fat in the skillet for flavor.

Reduce the heat to medium and add the finely chopped onion into the same skillet. Cook the onions, stirring frequently until they become soft and translucent, roughly 6 minutes. Once done, transfer them to a bowl to cool down a bit before using.

In a large mixing bowl, add your leftover mashed potatoes and use the back of a spoon to break them into larger crumbles. You want these to combine well but still have some texture. Add in the shredded cheddar cheese and the flour. Mix well to incorporate everything, using the spoon to press the potatoes gently so they absorb all that flour goodness.

Stir in the crispy bacon, sautéed onions, mustard powder, chopped scallions, and garlic salt (or garlic powder combined with salt). This is where the magic really begins. Mix until all ingredients are well-distributed.

Beat the egg until smooth, then add it to your potato mixture. Stir until everything is mixed together evenly. You should end up with a thick, cohesive mixture that holds together well.

Using a medium ice cream scoop or two large spoons, divide the mixture into about 8 equal portions. Each should be roughly 3 tablespoons. Roll each portion into a ball between your hands, then flatten them gently to form patties about ¼ inch thick. To absorb excess moisture from the surface, lightly press each patty on paper towels.

Line a baking sheet with paper towels and set a wire rack on top. Set this aside as your frying station.

Back to the skillet! Add 2 tablespoons of vegetable oil and heat until shimmering. Avoid crowding the pan; add as many patties as will fit comfortably. Fry each side for about 2 to 3 minutes until they turn golden brown. Gently flip them over with a spatula to brown the other side. Once done, transfer them to the prepared rack to drain while you repeat the process with the remaining oil and patties.

Once all the patties are crispy and golden, serve them immediately! They pair well with toppings like sour cream, chopped scallions, or even a drizzle of sriracha for those who love a bit of heat.