Make-ahead tip: Dressing and salad greens can be prepared a day ahead. Cover and chill separately.
To prepare dressing, combine first 6 ingredients in a large bowl; stir well with a whisk. Stir in onions.
To prepare greens, combine spinach, salad greens, and tarragon in a large bowl; add the dressing, and toss well. Serve salad immediately.
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