In a large saucepan, cover the limas with 4 inches of water and bring to a boil. Simmer over low heat for 1 hour, then add water to cover by 2 inches. Add the garlic, chopped fennel, onion and lemon zest. Boil over moderately high heat for 15 minutes, stirring often. Reduce the heat to low and simmer, stirring occasionally, until the lima beans and fennel become a thick and creamy puree, about 2 hours.