Peel, wash and finely dice the celery and carrots. Clean and wash spring onions and cut into fine rings. Peel and wash potatoes and cook in boiling salted water for about 25 minutes.
Heat the oil in a casserole, add the mince and fry vigorously while turning. Add spring onions, except 1 tablespoon for decoration, celery and carrots and fry while turning. Season with salt, pepper, sugar and paprika.
Stir in the tomato paste, sweat it on and deglaze with tomatoes. Bring to the boil, simmer for 4-5 minutes, chopping the tomatoes slightly. Season to taste with salt, pepper, paprika and sugar. Bring milk and fat to the boil in a saucepan.
Drain the potatoes, add the milk-fat mixture and mash with a potato masher. Season with nutmeg. Put the minced meat in an ovenproof dish and spread mashed potatoes on top. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 18-20 minutes.
In the meantime, wash, clean, quarter, seed and dice the tomato. Sprinkle the casserole with spring onions and tomatoes.