Preheat the grill to medium.
Place the cheese, mustard and egg yolks into a small bowl and mix well.
Place the bread slices onto a baking sheet and toast under the grill, turning once, until lightly golden-brown on both sides.
Spread the cheese mixture onto one side of each slice of bread and place under the grill until golden-brown and bubbling.
To serve, allow to cool slightly then place divide the rarebits between two serving plates and sprinkle with parsley.