In a small dry skillet over low heat, toast ground cumin 2 minutes or until fragrant, stirring occasionally. Add lemon juice and whisk well. Add oil in a thin, steady stream, whisking until blended. Add salt and pepper to taste. In a large salad bowl, combne greens with 1/4 cup dressing; toss to coat. Place on serving plates. Mix avocado, tomato, cucumber, and onion with dressing; toss to coat. Top each portion of greens with avocado mixture.