Drain the tuna and mash it in a bowl with a fork. Finely chop the onion and add with the remaining ingredients, stir and season to taste. If you like it creamier, use sour cream or creme fraiche instead of yogurt. It also tastes a little hotter with hot paprika, a teaspoon of mustard or a dash of Tabasco. This cold sauce on hot noodles is a dream, you can also use it as a dip or for sandwiches.