Cut the marrow into 1.5 cm rings, then remove the seeds and the soft fibrous parts in the middle.
Put the marrow rings in a dish and pour over them 3 tablespoon of oil, half of the salt and 2 minced garlic cloves. With your hands rub mixture well into the marrow, then leave to rest for about 30 minutes.
For the tomato sauce: slice the remaining garlic and fry it gently in half a tablespoon of oil with the fennel seeds. When the garlic begins to brown, add the chopped tomatoes, sugar, salt and sprinkle with a little pepper. Cook for about 8 minutes until you get a thick sauce then, using a hand-held blender, whiz until you get a creamy textured paste.
Heat the frying pan and fry the marrow slices on each sides until golden. Drain them on a paper towel then layer them on top of each other in a serving dish.
Pour over the tomato sauce. Season with pepper, then scatter some feta and basil leaves on top.