Cook spinach according to package directions. Drain well. Put drained spinach between paper towels and remove excess moisture by pressing with spatula. Chop spinach, set aside. Wash mushrooms and slice onions, set aside. Place chicken breast and onion in a medium sized sauce pan. Add enough water to cover and cook covered over medium low heat until white throughout. About 20 minutes. In a non-stick pan, saute mushrooms in olive oil until tender, about five minutes. Put spinach on bottom of broil safe dish. Sprinkle with nutmeg and pepper. Arrange chicken in single layer over spinach. Top with mushrooms. Enjoy!