Pre-heat the oven to 375° F.
Butter a 9"x13" baking dish.
Remove the blossom and stem ends of the squash, and slice it into 1/4" rounds.
Finely dice the onion.
Heat 1 tablespoon of the butter in a cast iron skillet, and sauté the onions and squash together until the onions are translucent and the squash is soft. This will take about 5 minutes.
Drain the squash and onions in a colander and pour into the bottom of the baking dish.
Whisk together the milk, sour cream, eggs, salt, pepper, and garlic powder.
Grate the cheese, and combine it in with the milk mixture.
Pour the mixture over the squash and onions.
Bake at 375° covered for around 30 minutes. Uncover and continue cooking for an additional 10 minutes until the casserole is set and brown on top.
**Optional Step** Melt the remaining 3 tablespoons of butter. Crush the crackers and sauté in the butter for around 2 minutes. Add the crumble to the top of the casserole prior to baking.
