Clean and wash spring onions, shake dry and cut into pieces of about 10 cm. Peel carrots, celery and garlic and cut into small cubes
Wash the meat, dab dry and cut in half horizontally. Heat the oil in a pan and fry the fillets for about 7 minutes, turning them over. Season with salt and pepper and remove
Put the diced vegetables into the pan and fry them for about 3 minutes. Add tomatoes and simmer for about 5 minutes. Season with salt, pepper, 1 pinch of sugar. Peel the potatoes and cut them lengthwise into approx. 0.5 cm thin slices
Melt the fat in a pot. Stir in flour and sweat briefly. Add stock and milk while stirring. Bring to the boil, boil for about 5 minutes, season with salt and nutmeg
Grease an ovenproof casserole dish. Put half of the potato slices in it, put half of the spring onion strips on top, add half of the vegetables and spread half of the sauce on top. Layer the remaining potato slices, the remaining strips of spring onion, the chicken slices, the remaining vegetables and the remaining sauce in the same way.
Bake in the preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see manufacturer) for approx. 40 minutes. Wash the basil, shake dry and pluck the leaves from the stalks. Remove the casserole and garnish with basil