Preheat oven to 200°C or 400°F.
Add onion, parsley and dill to mixer bowl. Chop for 3 sec/speed 5/MC on. Scrape bowl down.
Add oil and cook for 5 min/100°C or 212°F/speed 1/MC off.
Add chicken stock, mustard, pepper and lemon juice to mixer bowl. Put prepared potatoes in to deep steaming tray in place. Cook for 25 mins/steaming temp/speed 3 or until potato is cooked. While potato is cooking prepare your fish by cutting into large chunks and place in casserole dish with peas.
Set potato aside and add cream and corn flour. Mix for 10 sec/speed 4/MC on. Season with salt and pepper to taste.
Retain 100 g or 3.5 oz of sauce in the mixer bowl, and pour the remaining over the fish and peas.
Add potatoes, 1 tsp salt and pinch of pepper to retained liquid in mixer and mix for 5 sec/Speed 4/MC on or until desired consistency is reached.
Top fish and peas with mash potato and bake in the oven for 15 min or until golden brown on top. Let it cool on the bench for 10 mins before serving.