Preheat the oven to 180C/350F/Gas 4.
Heat the olive oil in a small ovenproof frying pan and fry the mushroom for 1-2 minutes on both sides, or until golden-brown. Keep warm in the pan.
In a small bowl, mash together the cheese, herbs, butter and garlic together until well combined, then use to stuff the mushroom. Sprinkle over the chopped onion and transfer the pan to the oven to roast for 5-6 minutes, or until the stuffing is bubbling and golden-brown.
Transfer the stuffed mushroom onto a serving plate and serve immediately.