Place oven bag in a 13- x 9-inch baking dish. Add flour; twist end of bag, and shake to coat.
Peel potatoes and parsnips, and cut into 1/4-inch-thick slices. Add vegetables, salt, and pepper to oven bag, and shake to coat. Add cream; squeeze bag to blend ingredients.
Close oven bag with nylon tie; cut 6 (1/2-inch) slits in top of bag.
Bake at 350° for 1 hour. Remove from oven, and let stand 5 minutes.