Spray a large saucepan or Dutch oven with vegetable oil. Place over medium-high heat. Add onion and garlic and cook until onion is tender, about 5 minutes. Add broth, lima beans, peas, salt, saffron, and pepper. Bring to a boil over high heat. Reduce heat, cover and simmer 10 minutes, or until lima beans are tender. Remove from heat and stir in uncooked couscous, tomato, and cilantro. Cover and let stand 5 minutes.