Boil water with salt and 3 dill stems. Place the fish in the boiling water and cook for about 10 minutes. Take out the fish and let it cool down.
Melt 30 g of butter in a pan, add flour and milk while stirring and a bit of salt. Stir until béchamel is formed. Transfer to a bowl and let it cool.
Peel and dice the apples. Finely cut up the rest of the dill
Whisk the remaining butter with bechamel in a bowl using a hand mixer, add apples, and dill. Mix and add the fish seasoned with salt, pepper, lemon juice. Mix everything and let it cool in the fridge.