In a large, heavy-bottom skillet over medium-high heat, place the bacon or chopped pancetta. Cook until browned and crispy, stirring occasionally (about 10 minutes).
Remove the cooked meat from the skillet, draining as much of the fat drippings as possible, and place on a paper towel-lined plate to finish draining.
Discard or reserve all but about 1 tablespoon of the rendered fat, and reduce the heat to medium. Add the chopped onion and cook, stirring frequently, until translucent and soft (about 6 minutes more). Remove the cooked onions and set them aside to cool.
In a large mixing bowl, place the leftover mashed potatoes and break them up into large crumbles on the bottom of the bowl.
Add the shredded cheese and flour blend, and mix to combine. Press down with the back of the mixing spoon so the potatoes absorb all the flour.
Add the cooked onions and bacon, mustard powder, scallions, and garlic salt (or garlic powder and salt), and mix to combine.
Add the beaten egg and mix until moistened and fairly uniform in texture.
Using a medium ice cream scoop or two large spoons, divide the mixture into 8 equal portions, about 3 tablespoons each.
Roll each portion into a ball between your palms, and then pat back and forth until you’ve created a patty shape about 1/4-inch thick. Press both sides of each shaped patty lightly on paper towels to absorb some of the surface moisture.
Line a baking sheet with paper towels, place a wire rack on top, and set the baking sheet aside.
Returning to the original skillet, add about 2 tablespoons of the vegetable oil and place over medium-high heat until the oil shimmers. Place as many of the shaped patties as will fit without any crowding in the hot oil.
Fry for about 2 minutes or until golden brown on the underside. Flip the patties gently, and let finish frying until golden brown all over (another 2 to 3 minutes).
Transfer the cooked patties from the pan to the prepared wire rack and baking sheet combo. Allow the cakes to drain briefly. Repeat with the remaining patties, adding the rest of the vegetable oil as necessary.
Serve immediately with toppings such as sour cream, hot sauce, chopped scallions and chives.
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Notes
For the flour – Nearly any flour will work here, as long as it's not grainy, which will add unwanted texture. Conventional all purpose flour or wheat flour will work just fine, as will almost any finely ground gluten free flour blend. My favorite is Nicole's Best multi-purpose gluten free flour, without added xanthan gum, for a crisp-tender result.