Spray the frying pan with low calorie cooking spray and sauté the mushrooms and spring onion for 2-3 minutes until soft. Place in a bowl.
Pre-heat the grill to high.
Allow the mushrooms to cool slightly, then add the grated cheese, mustard powder, Henderson's relish and egg yolk. Add a little freshly ground black pepper to taste.
Slice the muffins and place them cut side down on a baking sheet. Pop them under the grill and toast the bottoms until golden and crisp.
Turn the muffins over, but don't toast the top side yet! Reduce the grill to medium.
Spread the cheese and mushroom mixture evenly over the tops of the muffins and pop back under the grill for 2-3 minutes until the cheese has melted and gone golden.
Serve! We love this with some rocket and roasted or grilled tomatoes.