Dice the celery and chop the scallions.
Drain the tuna and add it to a bowl.
Add the mayonnaise, salt, and pepper, and mix gently.
Drizzle in the olive oil a little at a time, mixing to incorporate it into the tuna mixture.
Toast the bread under the broiler until lightly golden brown.
Spread the tuna mixture on one slice of toast, top with sliced cheese and tomato, and place under the broiler until the cheese is melted and bubbly.
Cut the tuna melt in half diagonally and serve.
