Preheat the oven to 180°C or 350°F.
With a knife, cut the top part of the marrow and set aside. With a spoon, hollow the centre of the marrow, making sure to not pierce through the sides and bottom.
Strain the removed marrow flesh, making sure to remove most of its water. Set aside both the filling and the hollowed marrows.
In a large bowl, mix together the minced meat, cumin, oregano, worcestershire sauce, minced garlic, chopped onion, tomato paste, beaten egg and some of the removed marrow filling chopped finely. You can also add some chopped hard boiled egg in the mixture.
Fill the marrows with the filling and cover the parmesan cheese. You can either leave them uncovered or top with the marrow top part.
Place the marrows in a large baking dish (glass or enamel are preferred). Around the marrows add the sliced onion and potatoes.
Fill the bottom of the dish with water, making sure that most of the potatoes are under the water. Drizzle with olive oil, salt and pepper and a generous helping of fennel seeds.
Cover the dish with foil and bake for an hour at 180°C/350°F.
Uncover the dish and let cook for further 30 minutes, until the potatoes are nicely golden brown.
Take them out from the oven and let them rest slightly before serving.