Prepare the Squash and Onions:
Preheat your oven to 350°F (175°C).
In a large skillet, melt the butter over medium heat. Add the sliced squash and diced onions. Cook for 10-12 minutes, stirring occasionally, until the squash is tender and the onions are translucent.
Mix the Filling:
Transfer the cooked squash and onions to a medium mixing bowl. Add the shredded cheddar cheese, sour cream, garlic powder, salt, and pepper. Mix until well combined.
Assemble the Casserole:
Spoon the squash mixture into a greased 9x13-inch baking dish. Spread it out evenly.
Prepare the Topping:
In a small bowl, mix the crushed buttery crackers with the melted butter. Sprinkle the mixture evenly over the top of the casserole.
Bake the Casserole:
Cover the dish with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the topping is golden brown and crispy.
Serve:
Let the casserole cool slightly before serving. Enjoy warm as a side dish to complement any meal!