Heat oven to 200C/fan 180C/gas 6. Twist and remove the stalks from the mushrooms. Place them, cap side up, on a small baking tray.
Melt the butter in a frying pan and, once it starts to froth, stir through the garlic and cook for 1 min. Blitz the slice of bread to fine crumbs and add to the pan. Cook, stirring for 2 mins, until lightly toasted.
Transfer to a bowl and cool before stirring through the finely grated Parmesan, chives and seasoning. Spoon into the mushroom caps, drizzle with a little olive oil and bake for 10 mins until slightly softened and the tops turn golden. Serve warm or at room temperature.