I like Tuna mayo with nice chunky slices of red onion.
Then again, I also like Egg mayo with cheddar cheese, red onion, lettuce and tomato on white bread. Crispy bacon with cheddar, lettuce, red onion and tomato on white...
70/30 mix of mayonnaise and salad cream, generous dollop of tomatoe purée (sundried if possible), freshly ground black pepper and a sprinkling of dried chilli flakes according to taste.