Mix garlic, herbs, salt, pepper and bread crumbs together in a small bowl. Rub mixture into rack of lamb. Cover bones with foil and roast lamb in the oven for 15 to 20 minutes for medium rare meat, or longer if desired. Let meat rest for 10 minutes before carving. Serve with Rosemary sauce.
Bring vegetable stock to a boil over medium high heat. Add rosemary. Reduce heat and simmer reducing liquid to 1/4 cup. Add demi-glace and simmer for 2 minutes. Dissolve the cornstarch in the port. Add to the simmering sauce, stirring until thickened. Serve under and on the side of lamb.