Cut the cured pork into small cubes. Peel and wash carrots and celery and cut into small cubes. Clean, wash and quarter leek lengthwise and cut into small pieces. Peel and finely dice the onion.
Heat the oil in a frying pan. Brown the smoked pork loin in it and take it out. Add the diced vegetables to the pan and fry for about 1 minute. Stir in tomato paste, fry briefly, then add the tomato cubes and stock.
Season the sauce to taste with salt, pepper and a little sugar and simmer gently for about 10 minutes. Grate cheese. Mix crème fraîche and sour cream and season with salt and pepper. Grease the casserole dish. Spread some sour cream sauce on the bottom.
Place 3 lasagne plates on top of each other. Layer tomato casserole sauce, sour cream sauce and again lasagne sheets until all ingredients are used up, finishing with sour cream. Sprinkle lasagne with cheese.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 40 minutes. Remove lasagne from the oven and garnish with basil.