- In a large non-stick skillet over medium heat, soften the onion in the oil. Season with salt and pepper. Remove from the heat and add the mustard. Set aside in a bowl. Clean out the skillet.
- Spread the mayonnaise on one side of each slice of bread. Divide the ham, onion mixture and cheese among half of the slices. Close the sandwiches. Butter the outside of the bread.
- In the same skillet over medium heat, brown two sandwiches at a time. Carefully flip the sandwiches over and cook until the cheese has melted, about 3 minutes on each side.