Rough-chop lettuce and place in large salad bowl. Small-dice tomatoes, about a half-inch thick. Drain and rinse garbanzo beans and add to salad mixture. Cut raw string beans to 1/4-inch and add to salad mixture.
Dressing: In a small mixing bowl, add lemon juice, Dijon mustard and garlic. Slowly stream in olive oil and season with salt, pepper and sugar to taste. Pour dressing over greens and toss together.