Bring a large pan of water to the boil, add asparagus, peas and both beans. Bring back to the boil, cook for 2-3 minutes, drain and plunge vegetables into cold water to cool (this stops the cooking process). Drain, pat dry.
Cut lettuces in half and arrange in a salad bowl, then tip vegetables over. Mix oil, vinegar and a little seasoning together with mint and chives. Drizzle over the salad and sprinkle with tarragon.
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