In small saucepan, bring vinegar and tarragon to a boil. Cool.
Blend mustard and vinegar mixture in deep bowl. Stir well.
Gradually add oil, beating until well blended.
Season to taste with salt and pepper.
Tear spinach into bite size pieces and place in a large mixing bowl. In medium bowl, generously drizzle lime juice over avocado cubes before tossing in salad to prevent browning and to enhance flavor. Add avocados, olives, onions and oranges to mixing bowl and toss together with salad dressing.