How to Make It
Place the muffin halves on a baking sheet, and broil 5 inches from heat for 3 minutes or until lightly toasted.
Combine the onions and the next 6 ingredients (onions through tuna) in a medium bowl. Divide evenly among the muffin halves. Top each muffin half with a tomato slice, and sprinkle with 2 tablespoons cheese. Broil 5 inches from heat for 4 minutes or until golden brown.