1) Place potatoes in a large saucepan, cover by 2.5-cm with cold water, add 1 tbsp of salt and bring to a boil over high heat. Cook until just cooked through but not soft, about 30 minutes depending on the size.
2) When cool enough to handle, slice crosswise into 1cm thick slices.
3) Preheat griddle to high.
4) Brush potatoes on both sides with oil and season with salt and pepper. Cook until lightly golden brown on both sides and just cooked through, about 1 1/2 minutes per side. Remove to a bowl.
5) Brush spring onions with the oil and griddle until slightly charred and softened, about 4 minutes. Remove and thinly slice. Add to the potatoes.
6) Whisk together the vinegars, mustard and honey in a bowl and season with salt and pepper. Slowly whisk in the olive oil until emulsified and taste for seasoning.
7) Pour the dressing over the potatoes, add the parsley and gently toss to combine.