Heat oven to 375°F Grease 9-inch square pan. Remove bones from chicken breasts. Do not remove skin. Loosen skin from chicken breasts.
In medium bowl, mix cheese, nutmeg and spinach until well blended. Spread one-fourth of mixture evenly between meat and skin of each chicken breast. Smooth skin over breasts, tucking under loose areas. Place chicken, skin sides up, in pan. Sprinkle with salt and pepper; drizzle with butter.
Bake uncovered 45 to 55 minutes or until juice of chicken is clear when center of thickest part is cut (170°F).