The butter is melted in a pan and add the mushrooms. Hold on medium heat, stirring, for about 5 minutes or until mushrooms are soft. Mushrooms are removed from the fire and keep warm. Mix the cream, eggs, 3/4 of the grated Parmesan, chopped onion, salt and pepper. Cooked and drained spaghetti put in a pan, add the egg mixture, mushrooms and chopped ham. Cook over low heat until the sauce thickens. Season to taste and serve sprinkled with remaining grated Parmesan.