Peel and wash the potatoes, quarter them lengthwise and put them in cold water. Clean and wash spring onions and chop them smaller if necessary. Melt the fat in a pot. Dust with flour and sweat briefly. Add milk while stirring, bring to the boil and simmer for about 2 minutes. Stir in 2/3 of the cheese and let it melt. Season to taste with salt, pepper and a little nutmeg.
Layer potatoes and spring onions in an oven dish and pour the sauce over them. Sprinkle the rest of the cheese on top. Bake in a preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 70-80 minutes until golden brown. (Use a sharp knife or fork to test the cooking time). Heat the oil in a pan. Leave the bacon in it until crispy, take it out and let it drain on kitchen paper. Remove the pan from the oven, let it cool down briefly and serve with the bacon
15 minutes waiting time