Heat the oil in a pan. Add the onion and garlic and cook until soft. Stir in the tomato purée, sherry, water and the cloves and season with salt and black pepper. Bring to the boil, cover and simmer gently for 45 minutes, adding more water if it becomes too dry.
Add the mushrooms to the pan, then cover and allow to simmer for about 5 minutes. Remove from the heat and allow to cool,still covered. Chill in the refrigerator overnight. Serve the mushrooms cold, sprinkled with the chopped fresh parsley.