I can remember eating these as a kid with Mom and Dad forty years ago. They are high up on the list of my favorite summertime comfort foods.
Spread the mustard onto one side of each of the bread slices. Place the luncheon meat onto half of the bread slices, then top each with 2 slices of Cheddar cheese, sliced tomato, and onion. Place the remaining bread slices, mustard-side-down onto the onions. Spread butter evenly onto the outside of each sandwich.
Heat a large skillet over medium-low heat. Place the sandwiches into the skillet, and cook until they are golden brown, then flip the sandwiches over, and continue cooking until golden brown and crispy on the other side, about 6 minutes per side. Allow the sandwiches to cool slightly before serving.
Per Serving: 775 calories; protein 34.8g; carbohydrates 33.7g; fat 56.5g; cholesterol 149.5mg; sodium 2065.6mg.