2 pounds small yellow-fleshed potatoes such as Yukon Gold
1 small red onion, finely chopped (1/3 cup)
2 tablespoons fresh lemon juice
3/4 teaspoon salt
1/2 cup sour cream
1/3 cup mayonnaise
1/4 cup chopped fresh dill
Cover potatoes with salted cold water by 1 inch and bring to a boil, then reduce heat and simmer, uncovered, until potatoes are tender, 15 to 20 minutes. Reserve 1/4 cup cooking water, then drain potatoes in a colander. While potatoes are boiling, soak onion in cold water 15 minutes, then drain in a sieve and pat dry. When potatoes are just cool enough to handle, peel and quarter, then toss with lemon juice and salt in a large bowl. Whisk together onion, sour cream, mayonnaise, dill, and reserved cooking water in a bowl until blended. Whisk in salt and pepper to taste, then add to potatoes and toss to coat. Serve warm.