Slice the potatoes in 1/2”/12mm thick rounds or semi-circles (depending on size of potatoes). There is no need to peel.
Trim and cut green beans into 1 1/2 – 2”/4-5cm pieces.
Slice onion with the vein into very thin sections. Slice radishes thinly. Finely chop parsley.
Cook the potatoes:
Bring a pot of water to a boil and cook potatoes until just tender but still holding their shape. Drain and allow to cool slightly. Cooking time varies, but will be about 15-20 minutes.
Blanch the beans:
Bring a small pot of water to a boil with 1 tsp/6g salt. Have a bowl with ice water ready.
Once the water is boiling, add the cut beans. Cook for 30 seconds. Immediately drain and submerge in ice water to stop cooking. Once cold, drain and set aside.
Make the dressing:
In a small bowl or mixing glass, combine all the dressing ingredients. Whisk until blended.
In a large bowl, combine the potatoes, beans, radishes, and onions. Pour over the dressing over and toss to combine.