In a large mixing bowl combine mashed potatoes and salmon while making sure no skin or bones end up in there as well, and break an egg into it.Add the lemon zest to the mixture, juice the lemon and pour it in as well along with chopped parsley and flour.Mix everything well, add a pinch of each salt and pepper.Scoop the mixture with an ice cream scoop to get even-sized cakes and roll them in the flour on your flour dusted board or plate, flatten them slightly to form a patty.Fry salmon cakes in hot olive oil until golden and crunchy on the outside.For the sauce mix yogurt, lemon juice, chopped chives and minced garlic in a small bowl.Add salt and pepper to taste. Let it sit for at least 15 minutes before serving for the flavours to blend.