Peel onions and potatoes. Cut the onion into half rings, potatoes into large cubes with a side of 2–2.5 cm. Boil 1 liter of water, add salt. Add onions and potatoes, reduce heat to medium and simmer, covered, for 10 minutes.
Wash the fish, cut into large pieces and put in a saucepan with vegetables. Add Santa Maria lemon flavored peppers.
Mix milk with flour, pour into a saucepan with soup. Bring to a boil and immediately remove from heat. Before serving, the dish can be garnished with chopped green onions or freshly ground black pepper.