Preheat oven to 200°c
Place mushrooms, stalk-side up, in a 7 cup capacity ovenproof dish. Drizzle over the olive oil and season to taste.
Heat butter in a medium frypan over high heat. Add the onion and chopped sage and cook stirring occasionally, for 4-5 minutes until onion has softened. Transfer to a large bowl. Add breadcrumbs and mix to combine. Add the chopped Taleggio cheese and egg, season and stir to combine. Spoon mixture evenly among mushrooms and season.
Roast fro 15-20 minutes until starting to turn golden on top. Place a slice of Taleggio over the top of each mushroom, return to the oven and roast for a further 10-12 minutes until cheese is melted and golden. Drizzle over extra olive oil and scatter over fried sage leaves to serve