Heat the oil in a large frying pan. Lightly fry the mushrooms in the oil for 2–3 minutes, turning once.
Put the mushrooms, gill side up, in an ovenproof dish (there is no need to remove the stalks). Fry the bacon and chorizo in the butter and garlic (if using) until the bacon has browned. Stir in the parsley.
Spoon the bacon mixture into the mushrooms and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 10–12 minutes or until the mushrooms are cooked through. Serve with thick slices of granary or wholewheat toast.