Put the milk and the broth in a saucepan, add 500 ml of water. Bring the liquid to a boil. Add the polenta and bring to the boil briefly. Remove from heat and let soak for 5 minutes. Finally stir in the butter.
Meanwhile, cut the onion and carrot into small pieces. Heat the oil in the pan and fry the minced meat in it. Add the onion and carrot. Pour in the tomatoes, reduce a little and then add water if necessary. Season the sauce and reduce for at least 12 minutes, season again to taste.
Grate the sliced cheese. Cover the bottom of a baking dish with a few spoons of polenta, sprinkle some cheese on top, then the Bolognese and some sliced cheese follow. Put a second layer in the pan, finish with polenta. Top with the torn mozzarella and some oregano. Bake in the oven at 180 ° C for about 20 minutes.