Heat just over half of the oil in a large nonstick pot over medium heat. Add the curry powder and heat for 1 to 2 minutes, until fragrant. Add the remaining oil, lamb, and celery to the pan and toss until the lamb is browned on all sides. Stir in the tomato paste, stock, and bay leaf. Increase the heat to high and bring to a boil, stirring often. Reduce the heat to medium-low, add the crushed tomatoes, and simmer for 50 to 60 minutes, until the meat is tender.