The night before, soak fava beans in 3 to 4 times their volume of cold water. Discard any favas that float.
The following day, drain, skin and cover with fresh water. Cook gently, with the garlic and cumin seeds, until the beans are tender (about 2 hours, depending upon the age and quality of the beans). Drain.
Puree the beans in a food mill or an electric blender. Stir in enough olive oil, and a little water, to give the puree a soupy consistency. Sprinkle with salt to taste and beat well. Heat just before serving with a little more olive oil and a sprinkling of crushed za'atar.
Serve with Arab bread and a bowl of "mixed spices" (ground cumin, cayenne and salt). Yield: 4 to 6 servings.