Step 1

In a large bowl, mix together the mashed potatoes, egg, flour, salt, and pepper until well combined. Fold in the cheese and green onion. The mixture should be moist but not wet; check to see if you can form a small amount into a patty without it sticking too much. If the potatoes appear too sticky, add more flour, 1 tablespoon at a time.

Step 2

Place the panko breadcrumbs in a shallow bowl. Divide the mashed potato mixture into 8 portions. Form the portions, one at a time, into 3 ½-inch disks. Dredge each disk in the panko, shaking off the excess. Set them on a plate.

Step 3

To a large cast-iron skillet over medium-high heat, add oil until it covers the bottom of the skillet by about ¼ inch. Heat the oil to 350°F. Fry the potato pancakes, in batches as necessary, until golden brown and crispy, 2 to 3 minutes on each side.

Step 4

Drain the pancakes on paper towels and sprinkle with salt. Serve hot, with a sprinkle of green onion and a dollop of sour cream.