In a small bowl, mix together mango, red onion, bell pepper, lime juice, sugar, salt, and pepper using a wooden spoon. Set aside until ready for use.

In a small bowl using a rubber spatula, mix the butter and curry together. Spread 2 tablespoons of the butter mixture on 1 side of each slice of bread. Place 4 slices of bread on your work surface, buttered side down. Spread the honey mustard on the other side of the bread. Place about 2 tablespoons of the mango salsa on the bread followed by the Swiss cheese. (You may need to fold the cheese slices to fit the bread.) Place the remaining 4 bread slices on top, buttered side up.

Add the remaining 1 tablespoon of curry butter to a nonstick skillet on medium heat for 2 minutes. Put the sandwiches in the skillet (in batches if necessary) and cover with lid and cook for 2 to 3 minutes, or until the underside is golden brown and the cheese has started to melt. Flip the sandwiches with a spatula and press very firmly, cook for 2 to 3 minutes, or until the cheese has melted. Turn once more, press with a spatula, and cook for 30 seconds. Remove from pan and let cool for 3 to 5 minutes. Cut diagonally and serve with extra salsa on the side.

Note: The salsa can be made 1 day ahead and refrigerated. Bring to room temperature before using.