Preheat oven to 350°.
Plae oven bag in a 13- x 9-inch baking dish. Add flour; twist end of oven bag, and shake to coat.
Add onion, vegetables, 1 1/2 teaspoons rosemary, 1 teaspoon lemon pepper, and 1/4 teaspoon salt to oven bag; squeeze bag to blend ingredients.
Sprinkle chicken breasts with remaining 1 1/2 teaspoons rosemary, 1 teaspoon lemon pepper, and 1/4 teaspoon salt. Arrange chicken over vegetables in an even layer.
Close oven bag with nylon tie; cut 6 (1/2-inch) slits in top of bag.
Bake at 350° for 40 minutes or until chicken is done and vegetables are tender.