Preheat the oven to 180C/350F/Gas Mark 4.
Heat up oil and butter in a large frying pan until butter has melted (addition of oil will prevent butter from burning) and add chopped onion.
Fry it on a medium heat, stirring from time to time for up to 10 minutes until soft and translucent. Set aside to cool slightly.
In a large bowl mix together breadcrumbs, sage, fried onion, marmite (if using) and some salt and pepper.
Bit by bit start adding some vegetable stock until your mixture is pliable and can be squeezed into stuffing balls.
Shape your stuffing into balls and bake on a baking tray in the preheated oven for approximately 30 minutes until browned and crispy.
If you decide to bake your stuffing in a dish, simply use the amount of stock as indicated above, transfer into baking dish and bake for approximately 35-40 minutes until golden and crispy.