Heat the oil in a large skillet over medium heat. Stir in the onion; cook and stir until the onion has softened and turned translucent, about 5 minutes. Reduce heat to medium-low, and continue cooking and stirring until the onion is very tender and dark brown, 15 to 20 minutes more. Add the garlic during the last 3 minutes of cooking.
Transfer the onions and garlic to a bowl and set aside. Turn the heat up to medium high and add the beef cubes. Cook, stirring occasionally, until browned on all sides, about 5 minutes. Drop in the tomato paste and reduce the heat to medium; allow the tomato paste to cook about 2 minutes, or until it begins to brown, before stirring.
Pour in the crushed tomatoes and return the onions and garlic to the pan. Mix in the parsley, basil, fennel, and salt and pepper. Reduce the heat to low and simmer, stirring occasionally, until the sauce is thick and the meat is tender, 6 to 7 hours.
