Melt the butter in a saucepan and add the cheese.
Cook over a low heat, stirring, until the cheese has melted.
Stir in the mustard, milk and salt and pepper to taste.
Slice the baps in half and toast the cut side.
Spread with the cheese mixture and top with the tomato slices.
Place under a preheated medium grill for 2 to 3 minutes.
Garnish with parsley.