Preheat the oven to 180°C and line a roasting tin with foil.
Bring a large pan of lightly salted water to the boil.
Scoop out marrow seeds and reserve 120g of the inner flesh.
Submerge marrow halves in boiling water, turn off heat, and let soak.
Sauté onions, garlic, and thyme in oil, then brown the beef mince.
Peel and chop fresh tomatoes if using.
Stir in tomato purée, paste, reserved marrow flesh, tomatoes, and parsley and simmer.
Cool the sauce slightly and stir in the beaten eggs.
Mix half of the grated parmesan and pecorino into the sauce.
Fill marrow halves with sauce, join them, wrap in foil, and top with remaining cheese.
Bake for 45 minutes at 180°C, then 5 minutes at 200°C to brown the cheese.
Rest for 10 minutes, slice into rings, and serve with extra garnish.