Preheat the oven to 200°C fan-bake.
Peel the potatoes and boil in salted water until soft.
Place the butter in the cast iron pot and melt over a medium heat.
Add the flour, stir and cook until it begins to froth.Take off the heat and stir through a ¼ of the milk. Combine until it is smooth.
Place back over a low heat and gradually add the remaining milk in small amounts. Stirring into a smooth consistency after each addition.
Once the milk has all been added leave on the heat to thicken.
In a small pan melt the butter and sauté the onion and garlic until soft and fragrant. Set aside.
Flake the fish into big chunks. If you are using canned fish - make sure to drain it first.
Place the onion and garlic, mustard, lemon zest, parsley and frozen peas in the cast iron pot with the white sauce. Gently combine.
Season with salt and pepper.
Next add the fish and combine once more.
Lastly top with the potato topping.
Use a knife or skewer to determine if the potatoes are soft enough to use as a topping. You want them to break in half when you skewer them.
Drain the potatoes and place on top of the fish pie in a rustic manner.
Sprinkle with the grated cheese and cook in the oven for 15-20 minutes.
