Prepare macaroni in boiling salted water according to the instructions on the packet. Peel the onion. Clean, wash, peel and finely dice the celery and carrots. Drain the pasta and let it drain.
Heat the oil in a large pan or pot and fry the minced meat until crumbly. Add onion, carrots and celery and fry briefly. Add tomato puree and sauté. Deglaze with 250 ml water and strained tomatoes, bring to the boil.
Stir in 1 tablespoon of stock, simmer for 10 minutes at low heat. Wash, clean and halve the tomatoes. Clean, wash and cut spring onions into rings. Add the tomatoes and spring onions to the mince mixture and cook for 2-3 minutes.
Season to taste with salt and pepper. Melt the fat in a saucepan. Sprinkle with flour and sauté while stirring. Gradually add milk and 250 ml water while stirring, bring to the boil. Add 1 tablespoon of stock, simmer for 5 minutes.
Melt the processed cheese while stirring, season to taste with salt and pepper. Grease a round casserole dish (26 cm Ø; 1.5 litres capacity). Layer macaroni and chopped sauce alternately in a circle.
Pour the béchamel sauce over it evenly. Sprinkle with cheese. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 35-40 minutes. Serve garnished with parsley and chervil.