Preheat the Oven to 435°F / 225°C
Cut the potatoes and place them in an oven-proof baking dish.
Let them cook for about 40 minutes, add the frozen fish onto the potatoes, after 20 minutes.
Melt the butter in a saucepan and add the finely chopped onion. Let cook for a few minutes.
Add the peas and slightly mash with a fork once they are heated.
Serve the fish with the potatoes, peas and ketchup, if desired.