Melt the butter in a cast iron, ovenproof frying pan over a medium heat, then add the mushrooms with the underside facing you, and allow to cook for 4 to 6 minutes until golden brown at the contact with the pan. Turn the mushrooms over and good for 3 to 5 minutes more. Flip back over and with the underside of the mushrooms facing you, sprinkle a little spring onion and garlic into each mushroom. Season with salt and pepper.
Bake in the preheated oven for 6 to 8 minutes, or until tender. Remove from the oven and serve.