Cooling the vegetables quickly helps to stop them from overcooking and will keep your salad crisp and green for longer.
Boil water in a large pan with a steamer attachment and fill a large bowl with ice-cold water. Steam the green beans for 3-4 mins, the courgettes for 2-3 mins, and the peas for 2 mins, until everything is just tender. (If you don’t have a steamer, just boil the vegetables in boiling salted water for the same times as above.) After cooking, immediately plunge vegetables into the ice-cold water to cool quickly, drain, then pat dry with kitchen paper.
For the dressing, whisk together the oil and lemon juice, stir in the parsley, mint and chives, then season. Take the dressing and vegetables to the picnic in separate containers, then toss gently together to serve.