Mix the mayonnaise with the mint leaves, mustard and 1 Tbsp water. Season with salt and pepper.
Put the potatoes in cold salted water. Bring to boil and cook until softened - about 10 minutes - then drain.
In the meantime blanche the beans and peas for 2-3 minutes in boiling water - or steam them in a bamboo steamer over the cooking potatoes. Rinse them with cold water and drain.
Combine the peas, green beans and potatoes in a large bowl. Peel and slice the avocado and drizzle with some lemon juice (so it doesn't turn brown).
Just before serving, add the avocado to the salad and pour over the mayo. Season to taste then mix gently and serve.