Bring the stock to the boil in a large saucepan. Heat the oil in a large heavy-based frying pan.
Season the sea bass fillets and cook, skin-side down, in the frying pan for 3–4 minutes until the skin is crispy. Turn over and cook for 1 minute on the other side.
Meanwhile, cook the fennel in the simmering stock 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 to taste.
Divide the vegetable broth between 4 bowls and sprinkle with the herbs. Top each portion with a sea bass fillet and serve immediately.