In large skillet, simmer chicken breasts, in a single layer, in chicken broth, covered until chicken is tender and juices run clear (approx. 20 min).
Remove chicken & cool slightly. Arrange in a 9x13 baking dish.
Reserve 1 1/2 cups chicken broth for sauce.
Spoon artichoke hearts over chicken.
In med. skillet, melt 2 tbsp butter;saute mushrooms and red pepper until all liquid has evaporated. Set aside.
In small saucepan, melt 1/3 cup margarine;blend in flour, salt and pepper. Gradually stir in reserved chicken broth and half-and-half. Bring to a boil, stirring constantly;boil and stir 1 minute. Gradually stir in Parmesan cheese until melted. Stir in sherry & rosemary;pour sauce over chicken. Sprinkle mushrooms and red pepper evenly over sauce. Refrigerate, covered several hours or overnight.
To Bake: Heat oven to 325 degrees. Bake covered, 1 hour; uncover and bake until heated through (about 15 min.).