Preparation Whisk together mayonnaise, sour cream, lemon juice, sugar, and pepper in a large bowl until sugar is dissolved. Pat mackerel dry and break into large pieces, discarding skin. Stir into dressing along with celery and apple. Toss arugula with oil and a pinch of salt in another large bowl. Divide arugula among plates, then spoon mackerel salad on top.
Preparation Whisk together mayonnaise, sour cream, lemon juice, sugar, and pepper in a large bowl until sugar is dissolved. Pat mackerel dry and break into large pieces, discarding skin. Stir into dressing along with celery and apple. Toss arugula with oil and a pinch of salt in another large bowl. Divide arugula among plates, then spoon mackerel salad on top.