Preheat the oven to gas 6, 200°C, fan 180°C.
Toss the vegetables in olive oil and place in a large roasting tin. Season and drizzle with the balsamic vinegar if using. Roast in the oven for 30-40 minutes until lightly browned and tender. Sprinkle with the chives and set aside.
Place the haddock fillets on a baking sheet, season and drizzle with a little olive oil. Roast in the oven for 7-10 minutes until just cooked through and flaking easily.
Serve the haddock with the roasted vegetables, sprinkled with basil leaves and a squeeze of lemon juice.
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