For maionese, combine yolks, lemon juice, garlic and mustard in a food processor. With motor running, gradually add oil in a thin steady stream until all is incorporated and maionese is thick. Makes about 300ml. Season to taste and refrigerate until required.
Meanwhile, bring a saucepan of water to the boil. Add eggs and cook until just hard-boiled (8 minutes). Remove with a slotted spoon, refresh under cold running water, peel, slice and set aside.
Combine tuna, shallot, capers, anchovy and 100ml maionese (or to taste) in a bowl, season and set aside. Remaining maionese will keep for 4 to 5 days refrigerated in an airtight container.
Spread one side of bread slices with butter and cut in half to form triangles, then, working with two triangles at a time, buttered side up, place a slice of egg on one half, then top with a tablespoonful of tuna mixture and a sprig of watercress, sandwich with remaining triangle and press edges lightly to seal. Repeat with remaining bread, tuna mixture and watercress and serve.