Preheat the oven to 180C/350F/Gas 4.
Remove the stalk from the mushroom and chop it finely. Heat the oil in a frying pan and gently fry the chopped mushroom stalk for one minute.
Add the breadcrumbs, parsley and garlic and fry for 2-3 minutes. Add the chopped spinach, season, to taste, with salt and freshly ground black pepper and fry for a further minute.
Place the mushroom onto a baking sheet and fill with the breadcrumb mixture. Transfer to the oven and bake for 6-8 minutes, or until the topping is golden-brown and the mushroom is softened.
To serve, place the stuffed mushroom onto a serving plate.