With smooth side of meat mallet or with rolling pin, pound each breast until about 1/3 inch thick. Sprinkle both sides with salt (if desired) and pepper; set aside. Heat oven to 350 degrees.
In large skillet, heat butter. Add onion and mushrooms; saute until onion is transparent, about 6 minutes. Stir in spinach, cheese and nutmeg to form stuffing; heat through.
Divide stuffing into 4 portions and spread evenly over each breast. Starting with short end of breast, roll up. Brush each breast roll with egg white; dredge with bread crumbs.
In 9 x 9 inch pan, place rolls seam side down. Cover and bake 15 minutes; uncover and bake an additional 10 minutes. To serve, cut into slices. Makes 4 servings.
Per serving: 276 calories, 41 grams protein, 9 grams fat, 238 grams carbohydrate, 238 milligrams sodium, 74 milligrams cholesterol, 0.3 gram fiber.
