In a small saucepan over medium heat, bring vinegar and tarragon to a boil; set aside to cool.
Whisk together the vinegar and mustard, then gradually add oil, beating until well blended. Season with salt and pepper.
Place spinach in large mixing bowl. In medium bowl, generously drizzle lemon juice over avocado cubes before tossing in salad to prevent browning and to enhance flavor.
Gently mix avocados, onions and kiwi into spinach. Pour in salad dressing and toss evenly to coat.