Photograph: Karen Thomas
Put the potatoes in a pan of cold salted water and bring to the boil. Simmer for 15 minutes or until tender. Drain and cool, then cut into quarters.
Cook the beans in boiling salted water for 2 minutes, drain and refresh in cold water, then squeeze from their skins.
For the vegetables, heat 2 tablespoons of olive oil in a large deep pan. Add the onion, fennel, garlic and a pinch of salt. Cook, stirring now and then, over a low heat for 20 minutes, uncovered, until the vegetables are soft and lightly golden.
Turn up the heat, add the courgettes and potatoes, and stir over the heat for 3-4 minutes. Add the stock, bring to the boil and simmer for 5 minutes.
Add the asparagus and cook for 5 minutes. Add the broad beans, then the spinach, in batches, and stir until the spinach has wilted.
Remove from the heat and stir in a drizzle of extra-virgin olive oil and the mint. Season to taste.
Meanwhile, heat a griddle or frying pan over a high heat, season the fish and brush lightly with the remaining olive oil. Cook, skin-side down, for 2-3 minutes, turn and cook for 1-2 minutes.
To serve, spoon the warm vegetables on to a platter, top with the sea bass and serve with extra-virgin olive oil and lemon wedges.
Get ahead: you can cook the vegetables a day ahead and reheat (but they'll lose some of their vibrant colour).