Dilled Lobster, Avocado, and Potato Salad with Horseradish Dressing Lieber

In a 5-quart kettle cover potatoes with salted cold water by 2 inches and simmer until just tender, 15 to 20 minutes. Drain potatoes in a colander and, while still warm, cut into quarters. Transfer potatoes to a large bowl and cool. Finely chop onion. Halve, pit, and peel avocado and cut into 1/2-inch pieces. Chop enough dill leaves to measure 1/2 cup. Cut lobster into 1/2-inch pieces and add to potatoes with onion, avocado, and dill. In a small bowl whisk together mayonnaise, sour cream, horseradish, vinegar, and salt and pepper to taste and pour over salad. Gently toss salad until combined well and season with salt and pepper. Chill salad, covered, at least 1 hour and up to 1 day.