Place potatoes in a large pot of cold water along with the salt. Bring to a boil then reduce heat and simmer until just cooked through but still firm in the center. Drain and let cool before slicing into rounds.
Put lardons in a large, cold pan and then warm over medium heat. Once some fat has rendered, increase heat to medium-high and cook until beginning to golden.
Reduce heat to medium and add onion. Cook until soft.
Add vinegar and sugar to pan, scraping and stirring to dislodge fried on bits and dissolve sugar.
Add oil and simmer briefly to form a thin dressing.
Add potatoes and herbs and gently toss until well coated. Adjust salt and serve warm, or at room temperature.