Cook the broad beans (if using fresh ones), sugar snap peas and asparagus in salted boiling water for 2–3 minutes, then plunge immediately into a bowl of ice-cold water. This is known as blanching. Drain and set aside. Pop the broad beans out of their skins.
Melt the butter in a large flameproof casserole over a low heat, add the spring onions and garlic and cook, without colouring, for 3 minutes, until softened. Add the stock and thyme and bring to the boil, then add the baby onions. Cover the casserole and simmer for 5 minutes.
Add the turnips, bring back to the boil, then reduce the heat and simmer for 6–8 minutes. Add the carrots and cook for 5–6 minutes. Season with salt, pepper and lemon juice. Add the beans, peas and asparagus and heat through. Serve garnished with the chopped herbs.