Bring 2 cups of water with 1 tablespoon of vegetable stock to a boil. Add the rice and cook covered on the lowest level for about 20 minutes. After about 10 minutes add the peas. Finally stir in approx. 1 tablespoon butter.
Knead 2 teaspoons of butter and flour together and place in the freezer for a moment.
Wash the fish, dab it, dice it, drizzle with lemon juice, season with salt and pepper. Pluck the rosemary from the branch, chop it up and sprinkle it on the fish.
Finely chop the onion and fry it in the oil in a non-stick pan, deglaze with the vegetable stock and cream and bring the sauce to the boil. Stir in the cold flour butter with a whisk until the sauce is thick. Season to taste with pepper, salt, lemon juice, sugar and mustard. Add the fish and let it simmer in the sauce for 8-10 minutes.
Even toddlers like to eat this dish when it is mildly seasoned!