Place lettuce and red onions in a salad bowl and season with salt and pepper. Set aside while dressing is made. Place orange juice and tarragon in a small pot and bring to a boil over high heat. Lower heat to a slow simmer and reduce by half. Remove the tarragon sprig and place juice in refrigerator to chill. When juice is chilled, place in a stainless steel bowl with all ingredients except for oil. Slowly drizzle oil into bowl while whisking ingredients until incorporated. Add salt and pepper to taste and set aside.