Stir mustard and aceto (1) in a small baking bowl, add oil drop by drop while stirring with a whisk until a thick sauce is formed.
Mix tuna and all ingredients up to and including chives with sauce using a fork, season.
Arrange tartar with lettuce, pour aceto (3) and oil over salad, add toast.
To serve, place a fishcake about 12 cm long on the plate, pour a quarter of the tartare into it, press lightly until smooth, lift the cakes precisely.