"Great way to use up roast lamb leftovers. The meat becomes so tender after being simmered for so long. My Irish husband ate 3 huge bowls of this the first time I made it. He said it was better than his grandma's back at home in Ireland."
Heat 2 tablespoons olive oil in large pot over medium-high heat. Coat lamb with flour and cook in the hot oil until the sides start turning golden, 1 to 2 minutes per side. Transfer to a plate.
Heat 1 tablespoon oil in the same pot. Add leeks and cook until transparent and some slices are golden, about 5 minutes. Stir lamb back into the pot. Cook for 2 to 3 minutes. Stir in beef stock, tomatoes, gravy, and beer. Reduce heat to low, cover, and simmer stew until vegetables are done roasting, 1 to 1 1/2 hours.
Place potatoes, carrots, parsnips, turnip, and garlic in a large glass baking dish. Coat with 1/4 cup olive oil and balsamic vinegar. Season evenly with rosemary, salt, and pepper.
Roast vegetables in the preheated oven, stirring every 20 to 30 minutes, until soft and starting to brown, 1 to 1/2 hours.
Stir hot vegetables carefully into the lamb stew. Bring to a boil. Mix 1 cup liquid from the pot with 1/2 cup boiling water and gravy granules in a small bowl until smooth. Stir into the stew. Dissolve cornstarch in 2 tablespoons water and add into the stew. Stir until stew thickens, about 5 minutes.