Heat the oil in a heavy large pot over medium-high heat. Sprinkle the lamb with salt and pepper. Toss the lamb with the flour in a large bowl to coat. Working in 2 batches, add the lamb to the pot and cook until brown, about 10 minutes. Using a slotted spoon, transfer the lamb to a bowl. Pour off the excess oil. Add the garlic to the same pot and saute over medium heat until tender and fragrant, about 1 minute. Add the wine and simmer over medium-high heat until reduced by half, stirring to scrape up any browned bits on the bottom of the pot, about 5 minutes. Return the lamb to the pot. Stir in the broth, tomatoes with their juices, and tomato paste. Cover partially and simmer over medium-low until the lamb is just tender, stirring occasionally, about 1 hour.

Meanwhile, cook the onions in a medium saucepan of boiling water for 2 minutes. Drain and cool. Peel the onions and cut off the root ends. Add the onions, potatoes, and carrots to the stew. Simmer until the lamb and vegetables are tender, about 25 minutes longer. Season the stew, to taste, with salt and pepper.