Peel the potatoes and cook them in salted water. In the meantime, rinse the fish in cold water, pat dry and season with the fish seasoning. Wash and finely chop the parsley. Peel the shallot, dice finely and sauté in butter in a pan. Mix flour with milk, add to the shallots and let everything simmer for a few minutes and season with salt and pepper.
Add the fish to the sauce and let it simmer at low heat.