1. Create the pancake mixture:
In a large mixing bowl, combine the mashed potatoes, shredded mozzarella, shredded gruyere, egg, flour, and chopped chives. Use a potato masher or wooden spoon to mix until everything is well combined. If the mixture is too loose to form patties, add flour gradually until the mixture holds together.

2. Shape the pancakes:
Scoop a heaping tablespoon of the potato mixture and shape it into a 1/3-inch-thick patty between your hands. Dredge each side of the pancake in breadcrumbs to coat. Place formed pancakes on a cutting board or tray and repeat the process with the remaining mixture.

3. Cook the pancakes:
Heat a large non-stick skillet over medium heat and add enough canola oil to lightly coat the bottom. Once the oil is hot, place the patties in the skillet in a single layer. Sauté the pancakes for 3-4 minutes on each side, or until they are golden brown and crispy. Repeat with the remaining patties, adding more oil if needed. You can use two skillets at once to speed up the process.

4. Serve:
Serve the mashed potato pancakes hot, garnished with a dollop of sour cream and a sprinkle of extra chopped chives if desired.