1. In a jar with a tight-fitting lid or a similar container, combine the oil, vinegar, parsley, mustard, salt, garlic, and pepper to taste. Cover and shake well. Refrigerate for at least 1 hour or until chilled. The dressing will keep in the refrigerator for up to a week.

2. In a large salad bowl, toss together the lettuce, spinach, watercress, cucumber, and onion slices. Remove the garlic and drizzle with the chilled dressing. Toss again to mix and serve immediately.