Place the Tuna steaks in large skillet and cover with the yellow onion slices.
Poach the Tuna until you can flake with a fork with minimal effort.
Set aside to cool & drain.
Mix all of the other ingredients in a large mixing bowl (note: start with a ½ cup of mayonnaise and adjust to taste).
Flake the Tuna.
Gently add the flaked Tuna to the mixed ingredients.
Chill & enjoy some awesome sandwiches.