Let the pollack fillet thaw and rinse briefly under running water and pat dry. Season with salt, pepper and lemon juice on both sides and dust with a little flour. Melt the butter in the hot pan and cook the pollack fillets.
In the meantime, dice a small onion and heat a good teaspoon of butter in a saucepan. Steam the onion until translucent. Then add a well-heaped teaspoon of flour and sweat a little.
Dissolve the vegetable stock in the 250ml hot water and deglaze the roux with it. Let something boil down. Then season with salt, pepper, mustard (more or less depending on your taste) and the cream. At the end I always add 1 heaped teaspoon of dill.
Serve the pollack fillet with the sauce. Potatoes or rice go well with it.