Peel and halve the onion and cut into thin rings. Place the onion in a perforated cooking container for a steamer and distribute it nicely. Peel the potatoes, cut into thick slices of your choice and pour over the onion. Put in the steamer for about 45 minutes, until they are just soft.
In the meantime, arrange the marinade. Mix all ingredients (vinegar, oil, mustard, sugar, salt, pepper).
When the potatoes are ready, let them cool for about 10 minutes. Put the potatoes and onions in a bowl, fold in the marinade, stir well. Season to taste with salt and pepper, add vinegar or oil if desired, add parsley and ideally enjoy a little warm.