Par-boil shells until al dente. In a large mixing bowl, beat cream cheese until fluffy. Beat in sour cream, parsley salt and pepper. Stir in chicken, In a small skillet, cook onions 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 cheese and 2 tbs. parmasan cheese and boursin cheese just until melted.

Spread about 1/2 cup cheese sauce in each of the two greased 11x7x2 dishes. Top with stuffed shells and remaining sauce. Cover and bake 350 deg. for 25 min. Uncover; sprinkle with remaining cheeses. Bake 10-15 min longer or until bubbly and cheese is melted.

You can substitute the manicotta with lasagna if you prefer.