Combine the mashed potatoes, both cheeses, crumbled bacon, flour, garlic powder, onions and egg in a large bowl and stir to combine. Chill the mixture, covered, for about 15 minutes, up to 24 hours.
Use an ice cream scoop with a trigger release handle to scoop potato mixture into small 2 inch rounds. Shape the rounds into 1/3 inch thick patties using your hands. Place the breadcrumbs in a shallow bowl and coat potato patties on both sides with breadcrumbs.
Lightly oil the bottom of a cast iron or non-stick pan with the vegetable oil over medium-high heat. Cook the potato patties for 3 to 4 minutes per side, until golden brown, and then place on a paper towel lined plate. Let cool slightly before enjoying. Dip in either sour cream or ranch dressing.