Clean and wash the leek and cut it into about 8 equally long pieces. Blanch in boiling salted water for about 4 minutes. Meanwhile melt 1 tablespoon of fat, add flour and sweat for about 3 minutes.
Quickly pour milk and stock into the roux while stirring. Bring to the boil and simmer for about 2 minutes while stirring. Cut 4 slices of cheese into fine strips and melt in the sauce. Season sauce with salt and pepper.
Cut the rest of the cheese in half lengthwise. Wrap the leek sticks individually first with 1 piece of cheese each, then with 2 slices of pork loin each. Place in a greased flat casserole dish and pour the sauce over it.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 20-30 minutes. In the meantime, prepare the spaetzle in boiling salted water according to the instructions on the packet. Drain the finished spaetzle, allow to drain well.
Melt 2 tablespoons of fat in a pan. Add the spaetzle and heat while turning. Season to taste with salt and nutmeg. Sprinkle the gratin with coarse pepper. Add the spaetzle.