Fill a large bowl halfway with ice and cold water; set a colander and clean towels nearby.
Cover potato pieces with cold water by 1 inch, add kosher salt, bring to a boil and lower to a steady simmer.
Simmer 12 to 18 minutes, checking at 10, until a thin knife enters with slight resistance and pieces stay intact.
Lift potatoes into a mixing bowl, keep cooking water and return it to a full boil.
Whisk vinegar, lemon, Dijon, garlic, honey, fine salt and pepper, then stream in olive oil.
Pour half the vinaigrette over hot potatoes, add onion, fold gently and cool uncovered 15 minutes.
Boil green beans 3 to 5 minutes, beginning checks at 3, until crisp-tender without a raw center.
Move beans to ice water for 2 minutes, drain and blot thoroughly on towels.
Fold dry beans, herbs, capers and remaining vinaigrette into warm, non-steaming potatoes.
Rest 20 minutes, taste potato and bean together, then adjust vinegar by the teaspoon or salt by a pinch.
