Cook asparagus in a skillet in a small amount of water for 6–8 minutes or until crisp-tender; drain well. Cover and refrigerate for at least an hour. In a jar with a tight-fitting lid, combine the vinegar, oil, mustard, sugar, tarragon, salt, and pepper. Shake well. Cover and refrigerate for at least an hour. Place asparagus in serving bowl. Top with ham, onions, and dressing. Serve with a slotted spoon.