Cut acorn squash in half. Roast them cut side up on a baking tray for 50 minutes or until the flesh is just tender. Let the squash cool for 20 minutes.
In a large frying pan over medium heat melt the butter; saute the onions. Stirring occasionally, cook the onions for 10 minutes or until they begin to brown at the edges. Mix in the garlic, coriander and nutmeg. Cook the mixture 2 minutes more; then remove the pan from the heat.
Spoon the seeds and stringy middle out of the squash and discard. Spoon out the flesh, chop it and add it to the onion mixture. Discard the skins. Heat and stir the squash-onion mixture, then season it with salt and pepper. Serve the squash hot.
If acorn squash is unavailable, use butternut squash instead.