Cut each squash in half and scoop out the seeds and membranes. Score a criss cross pattern in each squash and place on a baking tray cut side up. Drizzle with the olive oil, dot with the butter and season if desired. Cook for 40 mins on top shelf.
Place the mushrooms in an ovenproof dish with all the other ingredients and place of the shelf below the squash for its last 15 mins of cooking.