Spread softened butter over both sides of each slice of bread.
On half of the slices, spread a layer of mango chutney.
Top the other slices with cheese.
Cover the cheese with the onion and tomato slices, Season with salt and pepper, then top with the other slices, chutney side down towards the tomatoes.
Grill the sandwiches until the bread is golden brown and the cheese is melted, about 2 minutes per side.