I used to cook this as a vegetarian option for Sunday lunches in restaurants I've worked in and even the carnivores wanted some. If you're vegan, just don't add the butter and you're good to go.
It wins against a nut roast, hands down.
Preheat the oven to 180
Cut a marrow in half (width way and not length) and core it, then peel the outside. Salt and leave to stand and dry out for an hour as we need to remove moisture.
Cube carrots, sweet potatoes, parsnips (or whatever root vegetables you choose) and the mushrooms
Chop the onions and garlic
Add the oil to a large frying pan and when warm add the butter (if you want to go vegan, just don't add the butter)
Fry the onions and root veg until tender, add the garlic, mushrooms and oregano and continue to fry for another two minutes and take off the heat
Wash out the salt from inside the marrow and stuff with the tender fried veg and then join the two halves together again by wrapping tight with tin foil
Put on a baking tray and place in the oven for 40 mins