Peel the lemon. Separate the segments using a small, sharp knife. Discard any seeds. On a cutting board, chop the segments with the garlic to form a smooth paste. Transfer to a small bowl and, whisking constantly, slowly add the oil in a steady stream. Repeat with the cream. Season with the salt and pepper. Use a vegetable peeler to shave the carrots into ribbons. In a large bowl, combine the carrots, cabbage, onion, scallions, and dressing. Cover and refrigerate for at least 2 hours. Using scissors, trim 8 of the large inner lettuce leaves into neat cup shapes and place in a bowl of ice water. Just before serving, remove the lettuce cups from the water and shake dry. Spoon the coleslaw into the cups.