Heat your oven to 200°C/Fan 180°C/Gas 6. Halve the tomatoes. Tumble them into a large ovenproof dish. Halve the peppers. Scoop out the seeds and white bits. Slice the peppers into thick wedges. Add them to the tomatoes.
Peel and slice the onion into about 8 wedges. Peel and finely slice the garlic cloves. Add both to the tomato and pepper dish, along with the olives and half the oregano sprigs.
Drizzle the veg with 1 tbsp olive oil. Season with salt and pepper. Turn the veg a few times to coat in the oil and seasoning.
Nestle the cod fillets into the dish with the veg. Drizzle with 1 tsp olive oil each.
Put the dish in the oven for 25 mins. Roast till the veg are tender and a little charred and the cod is opaque and cooked through. The cod should flake easily when you press it with a fork.
While the cod and veg roast, pick the leaves off the remaining oregano sprigs. Halve the lemon. Slice one half into wedges. Keep the other half.
When the fish and veg are cooked, squeeze over the juice from the saved lemon half. Divide the fish and veg between two plates. Scatter the oregano leaves on top and serve with the lemon wedges.