Preheat the oven to 200°C, fan 180°C, gas 6. Mix the oil with the garlic and some salt and pepper in a large bowl. Halve the tomatoes. Cut the peppers and courgette into chunky pieces and the fennel into slim wedges. Toss the vegetables in the garlic oil then spread out in a shallow roasting dish and bake in the oven for 15 minutes.
If your cod pieces are whole, cut them in half so you have four equal pieces. Put the cod in the bowl that had the vegetables in, and turn to coat in the seasoned oil left in the bowl. Drizzle the balsamic vinegar over the vegetables, add the cod to the baking dish and return to the oven for 8-10 minutes, or until the cod is cooked through. Tear the basil leaves into the roasting dish, mix gently, then serve.