Preheat the oven to 200C/400F/Gas 6. Line a baking tray with baking parchment and brush the paper all over with vegetable oil.
Place the haddock fillet onto the baking tray, spread a little tomato ketchup all over the top of it, then sprinkle over the chopped olives, mushrooms, parsley and sweetcorn.
In the bowl, mix together the cheddar and breadcrumbs, then season with freshly ground black pepper. Cover the fish with the breadcrumb mixture.
Transfer the haddock to the oven and bake for 18-20 minutes, or until the fish is completely cooked through and the breadcrumbs are golden-brown. (The fish is cooked through when it is opaque throughout and flakes easily.) Serve with steamed green vegetables.