1) In a small bowl, mix together the mango, red onion, red pepper, lime juice, sugar, salt and pepper using a wooden spoon. Set aside until ready for use.
2) In a small bowl using a rubber spatula, mix the butter and curry together. Spread 2 tbsps of the butter mixture on one side of each slice of bread.
3) Place four slices of bread on your work surface, buttered side down. Spread the honey mustard on the other side of the bread. Place about 2 tbsps 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 four bread slices on top, buttered side up.
4) Add the remaining curry butter to a non-stick skillet on medium heat for 2 minutes. Put the sandwiches in the skillet (in batches if necessary), cover with the 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.
5) Remove the sandwiches from the pan and let them cool for 3 to 5 minutes. Cut diagonally and serve with extra salsa on the side.
Cook's note: The salsa can be made one day ahead and refrigerated. Bring to room temperature before using.