Clean, wash and drain the zucchini, carrots, celery and leek. Cut zucchini and carrots into cubes, celery and leek into slices. Peel onion and garlic.
Finely dice onion, press garlic through a garlic press. Heat olive oil in a large pan, crumble the minced meat into it and fry well. Finally, add onions and garlic and fry.
Coarsely chop the tomatoes in the tin with a long knife and add them to the minced meat. Stir in tomato paste and oregano and bring to the boil. Add vegetables and simmer covered for about 5 minutes. Season to taste with salt and pepper.
For the béchamel sauce, melt the fat in a pot and sauté the flour in it. Add milk and vegetable stock while stirring, bring to the boil and simmer for about 5 minutes. Season the sauce with salt, pepper and nutmeg.
Place a thin layer of vegetable mince sauce in a tall, greased casserole dish and cover with 3 lasagne dishes. Now alternately pour the remaining sauce and two more layers of lasagne into the dish. Finish with a thin layer of sauce.
Finally, pour the béchamel sauce over it and sprinkle with cheese. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 45 minutes. Serve garnished with fresh oregano.