In a large bowl, combine the mashed potatoes, whisked egg, flour, baking powder, garlic powder, onion powder, salt, pepper and chopped scallions. Mix until fully combined.

Scoop the mixture into about 10 portions and place them onto a parchment-lined baking sheet. Gently press each into a patty. Freeze for 10 minutes to help them firm up.

Stir together panko breadcrumbs, salt and pepper for the coating. Spread onto a rimmed plate or shallow bowl.

Heat oil in a large skillet over medium-high heat. Working one at a time, press each chilled patty into the seasoned panko to coat both sides.

Place 4 to 5 patties into the hot oil at a time. Fry for 1½ to 2 minutes per side, until golden and crisp. Transfer to a paper towel-lined plate. Repeat with remaining patties.

Stir together the sour cream and lemon juice in a small bowl.

Drizzle the lemon sour cream over the warm patties or serve it on the side for dipping. Garnish with scallions and enjoy.