Peel the onion and cut into fine cubes. Heat the oil in a pan, fry the onion and minced meat in it. Add tomatoes with liquid, tomato paste, crushed garlic, salt, pepper, oregano, thyme and reduce until the liquid has almost evaporated.
In the meantime, wash the zucchini and cut into slices.
For the sauce, heat the butter and sweat the flour in it. Deglaze with stock and milk while stirring and bring to the boil. Add processed cheese and season with salt, pepper and nutmeg.
Grease the baking dish, line with half of the zucchini, add the minced meat and distribute the remaining zucchini on top. Pour sauce over it. Sprinkle with grated cheese. Put the lid on the baking dish and put in the cold oven.