1 small onion
4 Tablespoons unsalted butter, divided (at room temperature)
salt and pepper, to taste
4 slices whole grain bread
2 slices Wisconsin Aged Swiss
2 slices Wisconsin Sharp Cheddar
2 slices Wisconsin Provolone
2 to 3 slices heirloom tomato
Peel and slice the onion and using one tablespoon of butter, sauté over medium heat with a little salt and pepper until caramelized, about 10 minutes.
Pre-heat the griddle to 400°F.
Butter each slice of bread on one side and place on the griddle buttered side down. Top two slices of bread with one slice of each cheese. Then add the onions and sliced tomato. Put the sandwich halves together and press down lightly. Turn the sandwich over once or twice to ensure even cooking.