Toast cumin in heavy small skillet over medium heat until slightly darkened, about 1 minute. Transfer to small bowl. Add mayonnaise and lemon juice; whisk dressing until smooth. Toss chicken, nectarines, celery, and onion in large bowl. Stir in dressing. Season to taste with salt and pepper. Spoon salad into lettuce leaves and sprinkle with almonds. Divide among 4 plates and serve.