Cook potatoes: Start in cold, well-salted water. Simmer gently until just fork-tender, 10–15 minutes. Drain. Let steam-dry 1 minute.
Whisk dressing: In a large bowl, whisk olive oil, lemon juice, zest, Dijon, vinegar, garlic, salt, and pepper.
Dress warm: Add warm potatoes to the bowl. Toss gently until coated.
Add mix-ins: Fold in onion (or scallions), celery, herbs, and optional capers/pickles.
Rest & serve: Rest 10–20 minutes to meld. Serve warm, room temp, or chill for later. Refresh with a squeeze of lemon and a drizzle of oil.