Mix Greek seasoning, lemon juice, vinegar and oil in small bowl with wire whisk until well blended. Gently stir in cheese. Refrigerate 15 minutes or until ready to serve.
Meanwhile, wash, dry and tear lettuce and spinach into bite-size pieces. Place lettuce and vegetables in large bowl. Just before serving, stir dressing; drizzle over salad. Toss to coat. Sprinkle with additional feta cheese, if desired.