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1
Preheat oven to 400 degrees F (200 degrees C).
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2
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.
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3
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.
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4
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.
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5
Roast vegetables in the preheated oven, stirring every 20 to 30 minutes, until soft and starting to brown, 1 to 1/2 hours.
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6
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.