If using the fava beans, bring a saucepan filled with water to a boil. Add the beans and blanch for about 1 minute, then drain and, when cool enough to handle, slip each bean free from its tough skin. If using frozen limas, reserve until needed.
Warm the olive oil in a saucepan over medium heat. Add the onion and sauté until translucent and tender, about 10 minutes. Add the garlic and s