Half-fill a bowl with water and ice. Add the cucumber, radish and onions.
Boil the asparagus, mangetout and peas in a pan of boiling water for 3 mins. Drain, then add to the ice bowl with the other vegetables. Leave for 5 mins, then drain and pat dry with kitchen towel. Toss all the veg in a salad bowl with half the coriander.
Whisk the dressing ingredients with 1 tbsp water and the remaining coriander. Drizzle over the salad.