Select a shallow, ovenproof casserole dish that is large enough to hold the fish steaks side-by-side without overlapping. Over medium heat, place half the oil in the casserole dish. Once the oil reaches a shimmer, fry the onion gently until softened, then add the garlic. Add the chopped peppers and chili flakes and stir-fry gently.
Season the fish steaks with salt and pepper. Heat the remaining oil in a frying pan and fry the fish steaks for two minutes on each side over a high heat. Add the tomatoes to the casserole dish and stir-fry briefly. Add the smoked paprika, salt, and pepper.
Add fish steaks into the sauce, moving the peppers into the spaces between them. Cover with the potatoes, pushing them down so they are as flat as possible. Add the white wine and bring to simmer.
Cover and cook very gently for about 45 minutes or until the potatoes are done. Taste for seasoning, sprinkle with chopped parsley, and serve immediately, straight from the casserole dish.