Preheat the oven to 200°C.
Cut the sweet potatoes into chunks and boil in salted water until tender. Once cooked, mash the sweet potatoes and keep it warm.
Place the fish portions in a large pan and add the milk. Season with salt and pepper and cook for 5 to 10 minutes on medium heat.
Remove fish portions from the pan and place them in a greased ovenproof dish. Keep the milk that the fish was cooked in.
Fry the onions and peppers in the olive oil until translucent. Add the garlic, mushrooms and baby marrows and cook for 5 minutes.
Add ½ a cup of the reserved milk and the cream to the vegetables and cook until the sauce has thickened. Season with salt and pepper. You can add some of the grated cheese to your vegetable sauce if you want to. If your white sauce is very thin, thicken it by mixing 1 t of flour or corn starch with a ¼ cup of cold water to make a slurry and add the slurry to your sauce. Cook for a few minutes until the sauce thickens.
Spoon the creamy vegetable sauce over the fish portions in the oven dish.
Top with the mashed sweet potato, spreading the mash evenly. Sprinkle with cheese and bake for 20 minutes until golden and bubbly.
Serve hot with steamed vegetables or a salad.