Peel the potatoes and blanch them in boiling water for 5-10 min. Once they cool, cut them into round slices. Melt 2/5 cup (100 g) butter and braise the flour in it.
Smear the bottom of a baking tray with the rest of the butter. Arrange 1/2 the potatoes, add the crumbled feta cheese, arrange the rest of the potatoes on top.
Pour on the sauce, sprinkle with the cheese and bake in a preheated 356°F (180 °C) oven for 20 min. or so.