1Heat oven to 325º. Grease rectangular baking dish, 11 x 7 x 1 1/2 inches. Cook and drain noodles as directed on package.
2Melt butter in 12-inch skillet over medium heat. Cook mushrooms and onion in butter, stirring occasionally, until tender. Stir in broth, 1 tablespoon of the tarragon, the salt and pepper. Heat to boiling; reduce heat to low. Stir in cream cheese and Swiss cheese until melted. Stir in chicken and pimientos.
3Cut noodles crosswise in half. Arrange 6 pieces, overlapping edges, in baking dish. Spread half of the sauce over noodles. Repeat layers of noodles and sauce. Cover and bake 25 to 30 minutes or until hot in center and bubbly around edges. Sprinkle with remaining 1 tablespoon tarragon. Let stand 10 minutes before cutting.