Heat 2 tbsp of oil in a large cast iron pot. Season the shanks with salt and pepper and fry them on both sides until golden brown. Do this in twice if the pot is not large enough.
In the meantime, finely chop the carrots, onion, celery and leek.
Roast the fennel seed in a dry pan until it turns light and becomes aromatic. Crush roughly in a mortar.
Remove the shanks from the pot and add extra oil. Fry onion, garlic and fennel seeds. Then add carrot, celery and leek and let it simmer for 10 minutes.
Deglaze with the port, add brown stock and tomato cubes. Season with salt and pepper and add thyme and bay leaf.
Put the shanks back in the pot, make sure they are under the sauce and let it simmer for at least an hour. Check regularly and add broth if necessary.
The dish is ready when the flesh falls off the bone. Remove the bone marrow, season with fresh pepper and fleur de sel.
Pull the meat apart and mix under the sauce.
Cook the pasta according to the instructions on the package.
Serve the Ossobuco on the pasta, finished with bone marrow, cheese and parsley.