Cut the fish into 2 cm wide pieces and cook them with the vegetables at low temperature in lightly salted water.
Then prepare a roux:
Melt the butter, then use a whisk to stir in the flour. When it is smoothly stirred, add 1-2 scoops of the cooking liquid and stir until there are no more lumps. It takes a moment, but it works. Gradually add the liquid until the sauce is a little too thick. Finally add the milk and stir too.
Add the fish and vegetables and season with salt, pepper, lemon juice and a little sugar.