Detach the caps of the large mushrooms, put, without chopping, into a frying pan with 1 tablespoon butter. Over a very moderate heat, cook without breaking. Chop the stems and the other mushrooms moistened with the lemon juice to prevent them from turning black. Fry gently in 1 tablespoon of butter until their water evaporates. Mince the ham, add to the mushrooms, keep aside. Make 2 cups of bÄchamel with the cream and the milk. Add the egg yolks, salt, pepper and a pinch of nutmeg. Take half of the bÄchamel sauce, add to the minced-mushroom mixture with a good pinch of grated GruyÅre cheese. Adjust the seasoning. Fill the mushroom caps, forming domes. Spread the remaining stuffing onto the bottom of an ovenproof dish. Top with the stuffed mushrooms. Coat with the remaining bÄchamel sauce, enriched with the remaining cheese and thinned with milk. Put into a hot oven to brown and reheat.
