Combine rosemary, 1 teaspoon salt, and coarsely ground pepper; sprinkle over entire surface of roast. Place roast, fat side up, in a roasting pan with a cover. Insert meat thermometer, being careful not to touch bone or fat. Bake, uncovered, at 475° for 20 minutes. Place chopped onion, carrots, parsley sprigs, and bay leaf around meat. Reduce heat to 375°; cover and bake an additional 40 minutes.
Sauté sliced onion in butter in a heavy skillet until tender. Set aside.
Place potatoes in a lightly greased 13- x 9- x 2-inch baking dish. Sprinkle with 1 /2 teaspoon salt and 1/4 teaspoon pepper. Top with sautéed onion. Pour 1/2 cup chicken broth over onion and potatoes. Set aside.
Remove roast from roasting pan, reserving vegetable mixture in pan. Place roast in baking dish over onion and potatoes. Bake, uncovered, at 375° on lower oven rack for 45 minutes or until meat thermometer registers 170°.
Add remaining chicken broth to vegetable mixture in roasting pan. Bring to a boil, and cook 5 minutes , stirring frequently. Strain through a sieve into a bowl; discard vegetables. Let broth stand 30 minutes; skim off fat.
Transfer roast, onion, and potatoes to a serving platter. Pour broth over roast. Sprinkle potatoes with chopped parsley.