Preheat your oven to 350°F (175°C). Grease a 9×13-inch casserole dish.
Cook the Squash and Onion:
In a large skillet or pot, bring water to a boil. Add the sliced squash and onion and cook for 5-7 minutes, or until tender. Drain well to remove excess moisture.
Mix the Filling:
In a large bowl, combine the cooked squash and onion, cheddar cheese, sour cream, mayonnaise, cream of chicken soup, garlic powder, salt, and pepper. Mix until evenly combined.
Assemble the Casserole:
Pour the squash mixture into the prepared casserole dish and spread it out evenly.
Prepare the Topping:
In a small bowl, mix the crushed Ritz crackers with the melted butter. Sprinkle the mixture evenly over the casserole.
Bake:
Bake uncovered for 25-30 minutes, or until the casserole is bubbly and the topping is golden brown.
Serve:
Remove from the oven and let cool for a few minutes before serving.
