Bring squash and onions to a boil and cook just until done. Drain extra well into a sieve.
Mash with a potato masher. Drain more if too watery. Pour from the sieve the squash-onion mixture back into the pot and add the rest of the ingredients.
Pour into a casserole or other baking pan. Bake at 350 degrees for about 1/2 hour or until set. The pan you cook in determines how fast it will be done. If it is thicker in a smaller pan it will take longer than if it is spread out into a shallow pan. It should be golden brown on top.