Preheat a grill to a high setting.
In a small bowl mix the cheese, onion, Mevalia Bread Mix and black pepper until combined. Add the butter Worcestershire sauce and mustard, mix well.
Toast one side of the Mevalia Pane Casereccio under the grill
Evenly spread the mixture on top of the untoasted side.
Place low under the grill, cooking for 4-5 minutes until the cheese has melted.