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1
Place the unpeeled potatoes in cold salted water and bring to a boil. Cook until a skewer meets only slight resistance at the centre — they must be fully cooked but firm enough to slice without crumbling. Drain immediately.
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2
While the potatoes are still hot enough to handle with a cloth, peel and slice them into 5 mm rounds. Work quickly and consistently — thick or uneven slices produce uneven dressing absorption, some pieces over-dressed, others under-dressed.
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3
Render the bacon lardons in a pan over medium heat until crisp and the fat runs freely. Remove the lardons with a slotted spoon and reserve both the lardons and the liquid fat separately. In the same pan, warm the beef broth with the reserved bacon fat and whisk in the mustard and white wine vinegar to form a unified, glossy emulsion — the gelatin in the stock binds the fat and acid into a dressing with visible body and sheen. This is where the dish lives or dies: a dressing built from cold fat on cold potatoes produces a greasy, flavourless salad that no subsequent adjustment will rescue.
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4
Pour the hot dressing over the warm potato slices at once. Turn gently to coat — the warm potato starch leaches slightly from the cut surfaces and thickens the dressing further into a natural emulsion that clings to each round. Fold in the reserved lardons and the fresh herbs.
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5
Taste before adding any salt — the bacon, mustard, and broth will already have seasoned the salad substantially. Adjust with black pepper. Cover and leave to rest for at least 30 minutes at room temperature. The salad is ready when the dressing has been fully absorbed and no liquid pools at the base of the bowl — the flavours will have melded and the surface of each slice will appear glossy rather than wet.
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6
Serve at room temperature. Cold fat from refrigeration congeals and the characteristic silky texture is lost — the salad must never be served directly from the refrigerator.