In a glass or stainless steel bowl, make the vinaigrette by stirring together all the ingredients except tuna and bread. Season with salt and pepper to taste. Let stand for at least 1 hour.
Drain the tuna and gently separate into pieces. Arrange the tuna evenly over the bread slices, then spoon 1-2 tbsp of vinaigrette over each.
This recipe easily doubles.
Serve as part of your tapas spread with anchovy stuffed olives and lemon wedges.