Cover one side of each slice with butter (10 g per slice). Smear it evenly up the edges.
Place 2 of the bread slices, with the smeared side down, on a cold non-stick pan. Turn up the heat to medium high.
Place 50 g of mixed cheese on each slice along with 50 g of mozzarella. Sprinkle with a pinch of salt and cover with the other two bread slices, with the butter covered side facing up.
Grill for 3-4 minutes, until golden-brown, then flip and grill for further 3 more minutes.
Serve warm with some tomatoes, or salad of your choice.