Remove outer leaves of lettuce heads and discard. Cut off tops of leaves and discard as they are bitter.
Chop remaining inner leaves. Rinse in cold water, and if you have a salad spinner, spin lettuce to dry. If you don't have one, remove as much water as possible in a colander and then dry with paper towels. This will keep the flavor of the salad from getting diluted.
Whisk together lemon juice, vinegar, olive oil, salt and pepper in a small bowl and toss with salad greens. Sprinkle with chopped walnuts if desired.