Preheat the oven to 200°C (400°F).
Heat olive oil in a large skillet. Add onion and saute for 5-6 minutes until soft and translucent. Add garlic and saute for 1 more minute, stirring constantly. Add Italian seasoning and dried chillies and salt and pepper (to taste).
Add ground beef, pork, and sausage and cook on medium heat until browned (4-5 minutes). Add 1½ cans of canned chopped tomatoes (Reserve the remaining ½ can for later). Simmer for 15-20 minutes until most liquid evaporates. If the mixture remains watery, scoop out and reserve this extra water for later use.
Whilst the meat sauce is simmering, prepare marrows/zucchini for filling.If using round marrows, cut off top cap (and reserve) and scoop out the middle flesh part using a metal measuring spoon with a round-back. Reserve or freeze flesh for later use. Note: you should still have a thin layer of flesh around the marrow.If using zucchini, cut them in half (lengthwise) and scoop out flesh. Once meat is ready, transfer to a large glass bowl and allow to cool for 5-10 minutes. Add diced marrow flesh/pulp if using. Then, mix in 2 lightly beaten eggs with meat mixture.
Cover the bottom of you baking dish with reserved sauce water and remaining chopped tomatoes + some extra water. Note: You may not need to use all remaining tomatoes, depending on the size of your baking dish, but you need to cover the bottom with a thin liquid layer to prevent marrows from drying out/burning whilst baking.
Stuff marrows/zucchini with meat mixture. Fill all the way to the top and sprinkle with grated parmesan cheese and breadcrumbs.
If using round marrows: Place marrow caps at the side of each marrow. Bake for 35-45 minutes until marrows are tender and tops are slightly golden.If using zucchini: Bake for 25-40 minutes until zucchini is tender and tops are slightly golden.Check the marrows/zucchini half-way through the baking and add extra water if they are drying up too much. Bring out from oven, allow to cool for 5-10 minutes, and serve warm with mashed or roasted potatoes or steamed rice.