Prepare the tuna salad by combining the tuna packet, mayo, relish, mustard, onion, and chopped egg if desired.
To make the sandwich place a slice of cheese on the bread, and then place a small scoop of tuna salad onto the cheese and spread it out evenly. Top with other slice of bread.
Drop a tablespoon of the butter or margarine (or non-stick spray) in a medium-hot skillet. When the butter has melted drop the sandwich into the skillet, rub the sandwich around to coat it with melted butter, then immediately flip it over. Slide the sandwich around the skillet to coat the bread with the remaining butter.
Cook on both sides for 2 to 3 minutes or until both sides have browned, and the cheese has started to melt.