Whisk lemon juice, mustard, shallot, 1 teaspoon tarragon, ½ teaspoon salt, and ¼ teaspoon pepper together in large bowl. Whisking constantly, slowly drizzle in oil. Add watercress, apples, and fennel and gently toss to coat. Season with salt and pepper to taste.
Step 2
Divide salad among plates and top with flaked mackerel. Drizzle any remaining dressing over mackerel and sprinkle with remaining 1 teaspoon tarragon. Serve immediately.