Toast the bread on both sides under the broiler, set at it's highest setting. While the bread is toasting, grate the cheese with a rotary grater. Place the mustard, pepper and butter in a bowl and mix well with a small spatula or a table knife. Divide the cheese mixture in half, then spread one half on each slice of toast. Carefully put the cheese covered toast under the broiler and leave it for 1 to 2 minutes until golden brown. When the cheese melts, remove carefully.