This is the way that I make mine, so that I do not have several bowls and dishes to wash at the end of the recipe! I end up using only one pot, one collander, one measuring cup, one spoon and one baking dish.
Preheat your oven to 350 degrees.
Cover squash and chopped onion with water in a pot and boil over medium heat until the squash is tender.
Drain squash and onion in the collander, and return to the pot. (I use the same pot, removed from the heat!)
Place the butter into the squash and onion mixture and allow it to melt from the heat of the hot squash and onion.
To the pot, add the salt, pepper, cheese, seasoning salt and crush the cracker crumbs by hand into a measuring cup and then pour them into the pot. Measure the milk in the same measuring cup that held the cracker crumbs. Add the egg to the measuring cup, and using a spoon, stir gently. Add this to the pot.
Mix everything in the pot until incorporated, but not so much that you completely mush up the squash. The end result will be a little runny, but it will firm during cooking.
Place everything in to an ungreased casserole dish using the same spoon that you used to mix your egg mixture.
Bake at 350 degrees for 30 - 40 minutes. Let stand at least five minutes before serving.