Preheat the oven to 190°C (Gas Mark 5 / 375°F).
Heat the oil in a frying pan, and add the diced onions, garlic and mushrooms. Cook over a medium heat for about 5 minutes, until the vegetables are soft and any excess liquid has evaporated.
Transfer the vegetables to a food processor, along with the remaining ingredients. Season generously, and blitz until the mixture just comes together.
Transfer the stuffing mixture to a baking dish, and bake for around 40 minutes, until crispy on top. Serve warm.