Kook de aardappelen: Boil potatoes in salted water until soft. Drain, then mash with butter, milk, and salt. Set aside.
Prepare the fish: Lightly poach the fish in water or milk for 5–7 minutes until just cooked. Drain and flake into chunks.
Make the sauce: In a pan, sauté onion in butter until soft. Stir in flour and cook for 1 minute. Gradually add milk while stirring until smooth and thickened. Season with salt, pepper, herbs, and mustard if using.
Combine filling: Add flaked fish (and peas/sweetcorn if using) into the sauce and mix gently.
Assemble the pie: Pour fish mixture into a baking dish. Spread mashed potatoes evenly on top.
Bak: Bake at 180°C for 25–30 minutes, until golden on top and bubbling at the edges.