To make vinaigrette, thoroughly whisk vinegar and salt/pepper in a small bowl. Add the Olive Oil in a slow stream while continuing to whisk until it is emulsified. Adjust vinegar and oil ratio to your taste and set aside. Arrange spinach and onion on 4 chilled salad plates. Top spinach with crumbled feta, tomatoes and berries. When ready to serve, drizzle the vinaigrette over the salads. Top with walnuts or additional feta cheese, if desired.