Cook the spaghetti in plenty of boiling salted water for 10-12 minutes. Wash the leek, dice the white end finely, cut the green part into fine strips. Heat the fat in a pot and fry the white leek in it.
Deglaze with the milk. Dice the gorgonzola and melt in the milk with mascarpone. Stir in the sauce thickener and season with salt and lemon pepper. Add the green leek to the pasta and cook for about 1 minute.
Roast the pine nuts in a dry pan until golden brown. Drain the pasta and leek. Arrange on a plate and pour the cheese sauce over it. Sprinkle with the pine nuts.