Rinse the potatoes and boil for about 25 minutes. Drain, evaporate, peel and cut into slices (not too thin).
Thoroughly rinse the lentils in a sieve under running water, then cook for about 5 minutes in boiling salted water. Drain in a sieve and allow to drain.
Rinse, trim and cut the scallions into thin rings. Peel the garlic and chop finely. Mix all the ingredients together.
For the dressing, mix the vinegar and mustard, season with salt and pepper and drizzle in the oil, whisking until combined. Mix with the warm salad and serve warm.