Cut the onion and bell pepper into small pieces. Mix with the drained tuna in a bowl. Hard boil the eggs. Also cut the egg whites into small pieces and add to the bowl.
For the dressing, mash the egg yolks with a fork and mix with the yogurt. Add balsamic vinegar and mustard if you like and stir everything well once.
Add the dressing to the other ingredients and stir everything through once. Salt and pepper to taste.