This idea is from Nigel Slater, who in turn, got it from Nigella. It's a great idea! Get a large field mushroom and crush a clove of garlic over the top. Add a few chunks or slices of butter, some fresh parsley and salt & pepper, and place in a hot (220C) degree oven for about 20 minutes, until the mushroom has become soft and gooily blackened.
This is perfect as a vegetarian version of a steak sandwich, and some bread dipped in the garlic butter in the pan is incredible. It's also really good as an accompanying vegetable to some steak or other meat. I had it as part of a vegetarian dinner, with some leftover cauliflower cheese. It was so good I repeated a few nights later with some steak. It's easy hand-free cooking. Put it in, while you concentrate on doing something else, and reap the benefits later. Yum!