Prepare mushrooms: Heat 15g butter in a large pan over medium-high heat. Add sliced mushrooms and cook for 5-6 minutes until golden and moisture has evaporated. Add garlic and cook for another minute.
Add truffle flavour: Stir in truffle oil, cream, and parsley. Season with salt and pepper. Cook for 1-2 minutes until cream thickens slightly. Remove from heat and let cool slightly.
Assemble toastie: Butter one side of each bread slice with remaining butter. Place buttered side down in pan. On two slices, layer half the Marsh Extra Tasty Cheddar, then the truffle mushroom mixture, then remaining cheese. Top with other bread slices, buttered side up.
Cook toastie: Heat a large frying pan or sandwich press over medium heat. Cook toastie for 3-4 minutes each side until golden brown and cheese is melted. Press down gently with spatula.
Rest and serve: Let toastie rest for 1-2 minutes before cutting diagonally. Serve immediately while hot and melted.