Put parsley into mixer bowl, chop for 4 sec/speed 6/MC on. Set aside.
Without washing the bowl, put shallot into mixer bowl, chop for 3 sec/speed 6/MC on. Scrape bowl down and repeat.
Insert butterfly and add white wine vinegar, dijon mustard, salt and black pepper to mixer bowl. Set mixer to 2 min/speed 3/MC off and VERY slowly start drizzling oil in to mixer bowl. Set aside.
Without washing the bowl, weigh in 500 g or 17.6 oz water. Put quartered potatoes in to simmering basket and steam for 17 min/steaming temp/speed 2/MC on.
When potatoes are cooked, but still firm, remove the simmering basket from mixer bowl and rinse under warm water. Drain water completely.
While the potatoes are still warm, stir through the reserved dressing, onions, add salt to taste and garnish with reserved parsley. Serve warm.