Heat oven to 350°F. Lightly spray large cookie sheet with cooking spray.
Cut loaf of dough crosswise in half to make 2 loaves. Place loaves seam sides down and 3 inches apart on cookie sheet. With sharp knife, cut 3 diagonal slashes, 1/2 inch deep, on top of each.
Bake 23 to 25 minutes or until golden brown. Cool 2 minutes.
Meanwhile, in small bowl, mix tuna and mayonnaise until well blended.
Set oven control to broil. With serrated knife, cut loaves in half horizontally; place cut sides up on same cookie sheet. Spread 2 heaping tablespoons tuna mixture onto each half loaf. Top each with 2 cheese triangles.
Broil 4 to 5 inches from heat 1 to 2 minutes or until cheese is melted.