Peel and either steam, bake or boil potatoes. Mash and set aside to cool completely.
Sauté onions and garlic in olive oil until onion is just soft. Set aside to cool.
Drain tuna.
Mix drained tuna with half of the beaten egg mixture, sour cream, cheese, salt, onions/garlic. Add mashed potatoes, mixing until completely combined.
Cover and refrigerate for at least an hour, up to 1 day. *This step is optional but highly recommended for ease of handling. If you don't time, skip this step and move on to the next*
Pour a shallow amount of oil into a pan (cast iron is ideal) and heat over medium high heat. Until oil is shimmery and hot. Reduce heat to medium.
Grab a plate or cookie sheet and line with paper towels or brown paper sack.
Using a spoon or ice cream scoop, measure out approximately 1/3 cup of the potato mixture. Flatten with hands (wet hands in cool water if the mixture is sticky) and dip into the flour.
Next dip the flour-coated potato cake into the beaten egg.
Then dip the coated potato cake into the bread crumbs, being sure to coat all sides evenly.
Carefully place into the prepared pan and fry for about 3-5 minutes. Flip and fry another 3 minutes, or until deep golden brown.
Remove from pan and let drain on paper towel/paper sack. Keep in a warmed oven while cooking the other batches.