Mix the onion, yogurt, garlic, vinegar and salt and pepper, mix well.
Heat the oil and soften the other onion and garlic for 10 minutes.
Mash the cooked warm potatoes, I leave the skin on.
Add the drained tuna to the warm potatoes, then add the onions and garlic and mix well.
Season well with salt and pepper, add a little more oil to moisten slightly.
Divide the mixture into 4 or 8 equal pieces, or you could make more, smaller fish cakes if you prefer. Then chill well.
Flour each cake well.
Heat 1cm of vegetable oil in a non-stick frying pan and cook the fish cakes for about 4-5 minutes on each side to colour nicely and warm through.
Adding a little extra oil makes the fish cake softer and more moist when cooked.
Oil in the fish cake helps to stop the cake becoming dry.
Fresh tuna can be substituted for canned if you prefer, just gently fry or microwave first to cook through.