In a large mixing bowl, beat cream cheese until fluffy. Beat in the sour cream, parsley, salt and pepper. Stir in chicken. In a small skillet, cook onion and mushrooms in butter until tender; add to chicken mixture. Stuff into manicotti shells.
In a large saucepan, melt the 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 11x7x2 in. dishes. Top with stuffed shells and remaining sauce. Cover and bake at 350 degrees for 25 minutes. Uncover; sprinkle with remaining cheeses. Bake 10-15 minutes longer or until bubbly and cheese is melted.