Clean the spinach and cook in salted water for 5 minutes. Drain. Chop the onion and garlic and sauté in 2 tablespoons of butter until soft. Mix 3/4 cup of breadcrumbs with chopped onions, garlic, mushrooms, chopped spinach, 2 tablespoons of parsley, chives, rosemary, thyme, salt and pepper.
Mix well and spread evenly in inner surface of the lamb. Roll, securing with string every few inches. Mix the remaining breadcrumbs with the rest of the parsley, salt and pepper and roll the meat in it, covering the surface. Melt the other 2 tablespoons of butter and dribble over the meat. Roast uncovered in an oven preheated at 350ºF. for 30 minutes per pound. Let the meat cool for 20 minutes before slicing.
Use leftover cooked rice instead of breadcrumbs, for the stuffing. For an unusual taste try 3/4 cup of chopped pistachio nuts instead of mushrooms and spinach. And don't be shy with herbs.