Slice the mushroom into 1-2" long pieces.
Hollow out the mushrooms.
Toss the hollowed out mushrooms in olive oil and bake until golden brown.
Meanwhile, pan fry the scraps from the hollowed out mushroom with with salted butter, rosemary, and garlic.
Once the pan-fried mixture is golden brown and fragrant, set aside and let cool.
Purée into a paste.
Fill the hollowed out mushroom with the mushroom paste, then top with caramelized onions and chives.
