Clean and wash the leeks, cut them in half lengthwise, cut the halves into long thin strips and set aside. Coarsely chop the walnuts, roast them briefly in a dry pot and take them out.
Peel and finely chop the onion. Heat the fat in a pot and sauté the onion. Add flour and sweat. Deglaze with milk and cream while stirring. Let it boil well. Crumble the gorgonzola and add to the sauce.
Season to taste with salt, pepper and lemon. Let simmer for another 5 minutes. Stir occasionally. Meanwhile cook the noodles in plenty of boiling salted water for about 5 minutes. After 3 minutes add the leek strips.
Drain the pasta and leeks and fill onto plates. Pour the gorgonzola sauce over it and sprinkle with the walnuts. Garnish as desired with lemon wedges and dill flags.