Bring a large pot of water to the boil and blanch the potatoes, drain and carefully slice in half.
Bring a large pot of salted water to the boil. Drop in the green asparagus, add the broad beans, peas, soy beans and spring onions and cook for a couple of minutes. Drain and run under cold water to stop them cooking but not to cool them.
Spread on a tray with the potatoes. Sprinkle with a little olive oil and season with salt and pepper.
Put the purple asparagus in a bowl and sprinkle with lemon juice and salt to cure and soften.
For the dressing, spoon the mustard and vinegar into a bowl and whisk until blended together. Slowly add the oils, still whisking, it should have a thick consistency, like a sauce. Season with a good amount of salt and black pepper.
Mix all of the vegetables with the shallots, mint, chervil, watercress and pea shoots and pour over the dressing.