Bring the stock to the boil in a saucepan.
Heat the oil in a frying pan. Season the sea bass with salt and pepper and place, skin-side down, in the pan. Cook for 3–4 minutes until the skin is crispy, then turn the fish over and cook for 1 minute on the other side.
Meanwhile, place the fennel in the stock and simmer for 3 minutes, or until it is just starting to become tender. Add the asparagus, peas and broad beans to the pan and cook for a further 1–2 minutes. Season the broth with salt and pepper.
Divide the vegetable broth between 4 bowls and sprinkle with a few torn mint and basil leaves. Top the dish with the pan-fried sea bass and reserved herby fennel tops and serve.