Preheat oven: Set oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter or cooking spray.
Sauté squash and onions: In a large skillet over medium heat, melt 3 tablespoons of butter. Add sliced squash and diced onion. Cook for 10–12 minutes, stirring occasionally, until tender and lightly golden. Remove from heat and allow to cool slightly.
Prepare the filling: In a large mixing bowl, stir together the sour cream, cream of chicken soup, beaten eggs, cheddar cheese, garlic powder, paprika, salt, and pepper until smooth and fully combined.
Combine with squash: Fold the sautéed squash and onions into the mixture and stir gently until evenly incorporated.
Transfer to baking dish: Pour the squash mixture into the prepared dish, smoothing the top with a spatula.
Make cracker topping: In a small bowl, mix the crushed Ritz crackers with 3 tablespoons melted butter. Sprinkle evenly over the top of the casserole.
Bake: Place in the oven and bake for 30–35 minutes, or until the top is golden brown and the casserole is bubbling at the edges. Let sit for 5 minutes before serving.
