Heat the oven to 200°C. Clean the mushrooms and cut larger pieces into smaller ones. Place the mushrooms in a pan and add the unpeeled cloves of garlic, sliced onion, oil, butter, rosemary and sage. And salt and pepper and mix the contents thoroughly. Place the pan in the oven and bake for 30–45 minutes. Meanwhile, boil the potatoes until they are soft (20 minutes). Add butter and milk, salt and pepper and mash them. Serve with the baked oyster mushroom and garnish with green leaves.