Wash and dry the lettuce, leaves and herbs. Tear larger leaves into bite-size pieces and tip all the greenery into a large salad bowl.
Put the garlic, onion, parsley and watercress or rocket into a mini chopper or food processor. Blitz until finely chopped. Add the vinegar, mustard, salt and pepper and blitz briefly. Add the oil and blitz until thickened. Drizzle over the salad and toss until the leaves are just coated. You might not need all the dressing, but it will keep in the fridge for a few days.