In a large skillet, saute the mushrooms, onion and garlic in butter until tender. Stir in the spinach and red pepper; cook for 3 minutes. Remove from the heat; stir in the Swiss cheese, bread crumbs, lemon zest, salt and pepper.
Carefully loosen chicken skin on one side of each chicken breast half to form a pocket; stuff with spinach mixture. Place in a greased 15x10x1-in. baking pan. Bake, uncovered, at 350° for 50-60 minutes or until juices run clear.
In a small saucepan, bring wine or broth to a boil. Reduce heat; simmer until reduced to 1 cup. Slowly whisk in cream and mustard. Bring to a boil. Reduce heat; simmer for 2 minutes. Spoon over chicken.