Cut the tops off the marrows, scoop out the filling. Blanch the marrows for 30 seconds in hot water and chop up the filling. Fry the onion and garlic in 1 tbsp oil for 3 minutes to soften then add in the mince and brown. Add in the chopped marrow filling and mixed herbs, season well with salt and pepper and cook for 3 minutes to soften. Add in the tomato puree and cook for a minute before adding in ½ glass water and cook for just 2 minutes. Remove off the heat and stir in 2 tbsp Parmesan and the beaten egg. Lay the potatoes and sliced onion in an ovenproof dish. Season well with salt and pepper and sprinkle over the fennel seeds. Pop the marrows amongst the potatoes, sitting in well. Stuff the marrows with the prepared filling and sprinkle over the remaining 1 tbsp Parmesan. Drizzle the potatoes with a little olive oil and pour over the stock. Cook in the oven for 45 minutes or until the potatoes are cooked and the marrow filling is brown and crunchy on top.