Seasoned roast with salt, pepper, 4 tablespoons olive oil and generously sprinkled with chopped fresh rosemary.
In roasting pan, place 4 ribs celery, 2 quartered onions and 4 peeled carrots, split lengthwise. Cook at 375 degrees for approximately 1½ hours until meat thermometer reads 135 degrees. After the first 30 minutes, add 2 peeled and halved potatoes per person. Season potatoes with olive oil, salt, pepper and rosemary. Turn potatoes after 20 minutes.
Let meat rest. Deglaze pan with 2 cups merlot wine, 4 sprigs rosemary, 2 cups chicken stock and 2 cups beef stock. Simmer 10 minutes. Thicken with 2 teaspoons cornstarch dissolved in 2 tablespoons cold water. Bring jus to boil. Add slurry. Whisk. Bring back to boil. Serve with lamb and potatoes.