Preheat the oven to 200°C (approximately 400°F).
Cut a lid from the top of the squash and scrape out the seeds. Cut the squash crosswise into rings and place next to each other on a greased baking sheet. Stir together the olive oil, salt and pepper and spread evenly onto the squash.
Cook for 20 minutes until golden brown (it should still have a slight bite).
Meanwhile, peel the onions and cut into thin rings. Melt the butter in a frying pan and sauté the onions briefly. They should still remain crisp.
Remove the squash from the oven and serve arranged with the onions, garnished with basil leaves.