In a medium bowl, mix the tuna, mayo, Dijon mustard (if using), chopped red onion, parsley, and lemon or lime juice. Add salt and pepper to taste. Mash with a fork until well combined.
Trim the crusts off the bread slices for that classic tea sandwich look. (Lightly butter the bread if you’d like.)
Spread a thin layer of tuna filling onto half the bread slices.
Cut each sandwich into quarters (either squares or triangles).
Add a baby lettuce leaf on top, then cover with the remaining bread slices. Use a toothpick to gently secure a cherry tomato on top of each sandwich.
Arrange your sandwiches on a platter. These are best served chilled or at room temp.