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, 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, whisk together the mustard and vinegar in a bowl, slowly adding the oils. 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.
Divide between four plates and dig in.