For the tuna spread, dice the onion very finely. Drain the tuna, mash with a fork, mix with the mayonnaise or jogonaise and the onions.
Season to taste with salt, freshly ground pepper and a little lemon juice. Depending on the consistency, stir in 1-2 tablespoons of sour cream.
Decorate as desired. E.g. with chives, dill, caper berries, young onion rolls, cress….