In a bowl mix tuna, red onion, mashed potato, lemon juice, black pepper, scallion, garlic, salt and parsley. Add beaten egg and mix to combine.
To form the croquettes, roll about 2 tbsp of mixture between your palms then place on a plate lined with parchment paper.
Place into the fridge for half an hour so that it can become firm.
Place flour into a plate then roll each tuna croquette in the flour to lightly coat.
Heat oil in a skillet; when oil is hot, fry the tuna croquettes over medium heat until golden brown on both sides.
Place on a plate lined with a paper towel to drain excess oil. Serve warm.
Saving some for later? Store in Rubbermaid Brilliance containers and put in the fridge!