COOK shells according to package directions.
HEAT oil; cook onion and mushrooms 5 minutes. In large bowl, combine mushroom mixture with chicken, 3/4 cup of the Parmesan cheese, sour cream, parsley, and nutmeg. Season to taste.
SPREAD 1/2 of the pasta sauce in bottom of 13x9x2-inch baking pan. Fill each shell with a rounded tablespoon of chicken mixture. Arrange stuffed shells in single layer and top with remaining sauce. Sprinkle with remaining Parmesan cheese.
BAKE at 350°F for about 30 minutes, or until tender.