Preheat the oven to 200ºС. Sprinkle the baking sheet with flour. In a large bowl, mix plain flour with wholemeal flour, add oatmeal, salt, soda and butter, cut into small pieces. Pour in milk, quickly knead the dough. Form a flat round cake with a diameter of 20 cm. Put the cake on a baking sheet, make deep cuts on it. Put in the oven and bake for 35 minutes. Remove the bread from the oven, cover with a dry towel and let cool slightly. Serve warm.