Heat a large skillet over medium heat and add oil.
Add onions and a pinch of salt; sauté for 10 minutes.
Lower heat to medium-low and cook, stirring occasionally, until golden and jammy, 45 minutes to 1 hour.
Finely chop caramelized onions and set aside.
Boil potatoes in salted water until tender, 20–25 minutes.
Drain and let cool; peel off skins and cut into chunks.
In a large bowl, combine lemon zest and juice, mustard, vinegar, salt, and pepper.
Whisk in 3 tablespoons olive oil until emulsified.
Add potatoes, caramelized onions, and fresh herbs to the bowl.
Toss to coat with dressing.
Taste and adjust salt if needed.
Refrigerate for a few hours or overnight before serving.