Start by preheating your oven to 350º F (175º C). Getting that temperature just right ensures an evenly baked casserole.

In a medium skillet over medium-low heat, melt 2 tablespoons of butter. Add the sliced squash and chopped onion, stirring occasionally. Cook until the squash softens and the onions become transparent, which should take about 6-8 minutes. This step brings out the flavors and gives the squash a delightful tenderness.

While your squash and onions are cooking, grab a large mixing bowl. Add the two lightly beaten eggs. Pour in the milk and whisk together until the mixture is smooth. Then, fold in the grated cheddar cheese, kosher salt, black pepper, and nutmeg if you decided to include it. This cheese-laden blend is the heart of your casserole, so make sure it’s well combined.

Once the squash and onions are cooked down and cooled slightly, transfer them into the egg mixture. Stir everything together until all the ingredients are nicely incorporated. You’re looking for a vibrant combination of veggies and creamy mixture.

Melt the remaining 2 tablespoons of butter in the same skillet used for cooking the squash and onions. Once melted, pour it into your squash casserole mixture and give it another good stir. The butter is what’ll keep things velvety.

Spray a standard 9x13-inch casserole dish with cooking spray. Pour the squash mixture into the prepared dish, spreading it out evenly. Then, take your crushed Ritz crackers and sprinkle them over the top in an even layer. This creates the ultimate crispy topping—it’s the best part!

Place the casserole in your preheated oven and bake for about 45 minutes. You’ll know it's ready when the top is golden brown and the center doesn’t jiggle when you shake the dish gently. Grab it from the oven, let it cool for about 3 minutes (if you can wait!), and serve warm.