Heat the oven to 220°C/200°C. In a roasting tin, toss the potatoes in the olive oil, season with salt and pepper, then transfer to the oven to roast for 25 minutes.
Meanwhile, heat a frying pan, add the chorizo and cook for 4-5 minutes until it has released most of its oil. Add the breadcrumbs, then cook for another 3-4 minutes until crisp. Transfer to a food processor with the lemon zest and manchego, then whizz to fine crumbs.
Add the peppers to the potatoes in the roasting tin, toss to coat in the oil, then roast for 15 minutes more.
Toss the cod loins in the lemon juice, then spoon over the chorizo crumb, pressing down to make a crust on top of each piece. Put the fish on top of the veg in the roasting tin, then return to the oven for another 10-15 minutes or until the fish is cooked through. Scatter over the olives and parsley and serve the traybake.