For an extra layer of flavor, consider adding a thin spread of Dijon mustard to the bread along with the mayonnaise. Experiment with different types of cheese! Gouda, Gruyere, or even a sharp cheddar would be delicious alternatives to havarti. Don't overcrowd the skillet; cook the sandwiches in batches to ensure even browning. Serve with a side of your favorite pickle-flavored snacks or a crisp green salad to complete the meal.