Wash the potatoes and boil in a pan of salted water and cumin for 25 minutes until tender.
Drain and peel while still hot. Allow to cool and cut into thin slices.
For the dressing, peel and finely dice the onion.
Blanch the onion briefly in boiling salted water.
Drain well. Add to the potatoes.
Heat the stock in a pan with the vinegar, salt, chilli and sugar. Remove from the heat and stir in the mustard.
Pour into a blender and blitz with the oil and a handful of potato slices.
Season to taste and stir into the potato salad. Leave to stand for 20 minutes. Separate into 2 bowls and use one for the variation.
Wash the lettuce, shake dry and cut into strips.
Cut the bacon into pieces and fry in a hot pan until crispy.
Mix the bacon and lettuce into the reserved potato salad. Season to taste and serve with the other salad.