Step 1Place the plums in a small bowl and pour the Port over them. Set aside.
Step 2Remove any tough silver skin from the lamb shanks by lifting it with the tip of a knife, then peeling and cutting it off. Rub the surface of each shank with the garlic cloves. Season with salt and pepper. Place the flour in a plastic bag. Add the lamb shanks one at a time and shake to coat.
Step 3Heat the olive oil in a large skillet over medium heat. Add the lamb shanks and cook until brown on all sides, about 25 to 30 minutes. Transfer the lamb from the skillet to a roasting pan large enough to hold the shanks in a single layer.
Step 4Add the onion, carrot, celery and minced garlic to the drippings in the skillet and saute about 5 minutes. Stir in the minced rosemary. Blend the tomato paste with 2 tablespoons of the chicken broth and stir in. Add the remaining chicken broth, beef broth and Merlot.
Step 5Drain the Port from the plums (reserving the plums) and add. Season the mixture with salt and pepper to taste. Pour the mixture over the lamb shanks. Add the soaked plums, the apricots and the bay leaf.
Step 6Cover the roasting pan, place on the middle rack of a 325-degree oven and braise for 1 hour, basting after about 30 minutes. Remove the pan from the oven and turn the shanks over in the broth. Braise for an additional 45 minutes to 1 hour, basting after 30 minutes, until the lamb is fork tender.
Step 7Arrange the lamb shanks on a platter and keep warm. Bring the sauce in the roasting pan to a boil over medium heat. Reduce the heat and simmer until sauce is slightly thickened, about 5 minutes.
Step 8To serve, spoon the vegetables and some of the sauce over the lamb. Pour the remaining sauce into a gravy boat. Garnish the platter with a few sprigs of rosemary.