In a medium bowl, combine all of the ingredients (except bread and cheese). Toss with a fork until well-mixed.
Place the bread slices on a broiler pan or baking sheet. Divide the tuna mixture evenly among the slices. Sprinkle the cheese evenly over the tuna mixture.
Place under the oven broiler and grill until the tops just begin to bubble (about 4 minutes). Be careful not to let the cheese brown too much, and you don't want the bread to burn.