Add fresh herbs to a dry mixer bowl. Chop for 2 sec/speed 8/MC on. Scrape bowl down and repeat if necessary.
Insert butterfly and add remaining ingredients, except for potato and spring onion. Blend for 10 sec/speed 3/MC on. Scrape out sauce and set aside in the fridge.
Without washing the bowl, add 300 g or 10.6 oz water and insert internal simmering basket. Put potato in the basket and cook for 17 min/steaming temperature/speed 3/MC on or until potato is just cooked but still firm. If using cauliflower, cook for 13 mins.
Allow potato to cool completely before stirring through sauce and seasoning to taste with salt and pepper. You will get the best results if you refrigerate the potato salad for a few hours before serving. Top with spring onion before serving.