Let the yeast swell briefly in some of the lukewarm water. Put the two flours in a bowl, add the oat flakes, sugar, melted butter, salt, yeast and the water and briefly work everything into a yeast dough. Until everything is well connected, this is a bit sticky dough. Cover the dough with cling film and let it rise overnight at room temperature. Make sure the bowl is big enough so that the dough will rise well.
On the next day, dust a casserole dish about 36 cm in size with flour. This can of course also be a round one. Loosen the dough from the edge in the bowl and shape it into an oval or round shape using a little flour, depending on the roaster. Very important: do not knead the dough any more!
Place in the roasting pan, cover and let rise in a warm place for an hour. It should have increased in volume again. In the meantime, preheat the oven to 250 degrees, if you want, sprinkle the loaf with oat flakes. Put the lid on the roaster and bake the bread for 45-50 minutes. Then carefully remove the lid and bake for another 5-15 minutes, until the bread has a nice brown color.
Take out and leave to cool completely on a wire rack, only then cut.
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