Boil potatoes/sweet potatoes in salted water until soft. Drain, remove skins and mash. Heat some oil in pan. Cook onions for a few minutes, then add minced meat. Cook until meat is browned, then add spring onions and carrots. Season with oyster sauce, salt and pepper to taste. Let the whole thing simmer on low heat for a few minutes. Add the cooked meat to mashed potatoes/sweet potatoes. Mix well. Let the whole thing cool and then leave it in the fridge overnight (or at least 2-3 hours). This will let everything to permeate nicely. Shape the croquette mixture into a preferred size or shape. Some people like to add cheese to their croquettes; if you’re one of them, now’s the time to do it. Just stick a piece of cheese or a cheese cube in the middle of your croquette. Coat croquette in flour, eggs, and then panko. Fry until golden brown.