Chop the onion finely and fry in margarine, sprinkle with flour and pour in the milk. Boil on low heat, stirring continuously until a smooth sauce. Season with salt, pepper, lemon juice to taste and a pinch of sugar. Add the eggs, cut into quarters and canned fish. Add diced or grated feta cheese. The resulting mixture is distributed in pots and bake in the oven until golden.