1. In a 6- to 8-quart pan, combine potatoes and 4 quarts water. Cover and bring to a boil over high heat. Reduce heat and simmer until potatoes are tender when pierced, about 20 minutes. Drain.
2. Meanwhile, in a large bowl, mix lemon juice, olive oil, mustard, and salt. Add potatoes, chopped onion, and dill and mix gently to coat. Serve warm, at room temperature, or chilled.