Wash and dry the mushrooms. Melt butter in a small pan then add mushrooms and cook at a medium heat for a couple of minutes stirring constantly. Add the garlic, continue cooking & stirring for another minute or so. Stir in oyster sauce and season with just a touch of salt and pepper.
Reduce heat to absolute minimum then cover with a tight fitting lid. Leave the garlic mushrooms to sweat for at least ten minutes, stirring occasionally.
Serve the garlic mushrooms with some of the gloriously wicked garlicky butter.