To prepare the sauce: soak 50 grams of mustard seeds in 100 mml of beer. Brew 20 ml of beer at high temperature to about half the quantity. Whisk in the cold butter. Sieve the sauce, add the mustard seeds and season with sugar and salt. Rinse the potatoes and stew them in their skins for about 30 minutes.
Separate the potatoes from the skin and mash them with a fork. Drizzle with two tablespoons of olive oil. Add salt and parsley bunches and keep warm.
Cut the cod fillet into four pieces. Heat the remaining olive oil in a frying pan and brown the fish on the skin side until crisp. Then place the fish in the heated oven at 200° Celsius, fan 170° Celsius, gas mark 3 for about four to six minutes.
Preheat the plates properly and heat the potatoes again. If they are too dry, add a tablespoon of tap water. Spread the mashed potatoes on four plates.
Tip: Feel free to use better olives – it pays off!