Combine tuna, mayonnaise and salt and pepper to taste and set aside. Spread butter on one side of each slice of bread. Spread tuna mixture over one slice of bread, butter-side down. Add cheese and tomato slices and add other slice of bread, butter-side up.
Cook over medium-low heat in a covered, pre-heated skillet, turning at least once, until bread is toasted golden brown and cheese is melted.
Serve warm with a cup of tomato soup.