In a large mixing bowl, beat cream cheese until fluffy. Beat in the sour cream, parsley, salt and pepper. Stir in chicken. (If you want a creamier texture, place the cream cheese, sour cream and chicken in a food chopper) In a small skillet, cook onion in butter until tender; add to chicken mixture. Stuff into manicotti shells. (To make this easier, put mixture into a baggie, seal it up, cut the tip off of corner of bag and pipe into tube.).
In a large saucepan, melt butter. Stir in flour and salt until smooth. Gradually whisk in milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Stir in 2-1/2 cups Monterey Jack cheese and 2 tablespoons Parmesan cheese just until melted.
Spread about 1/2 cup cheese sauce in each of two greased 11-in. x 7-in. x 2-in. dishes. Top with stuffed shells and remaining sauce.
Cover and bake at 350° for 25 minutes. Uncover; sprinkle with remaining cheeses. Bake 10-15 minutes longer or until bubbly and cheese is melted. Yield: 7 servings.