In a bowl, add the drained tuna, mayonnaise and pickle relish. Mix together. Spread one side of each slice of bread with butter and the other side with mustard. On top of the mustard side of two pieces of bread, divide the tuna and spread on bread. Cut tomato slices in half and place on top of the tuna. Top each with ¼ cup cheese. Place the remaining slices, mustard side inside, on top to create a sandwich. In a large nonstick pan, add the two sandwiches, cheese side up. Cover with a lid and cook on medium-high heat about 2-3 minutes until bottom is golden. Using a spatula, turn each sandwich over. Flatten a little with the spatula. Cover, and cook about 1-2 minutes until golden. Turn sandwiches over again and cook about 30 seconds more until cheese is melted.