Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish.

Bring salted water to a boil in a large pot. Add sliced yellow squash and chopped onions, cooking for about 5 minutes until tender yet firm. Drain well.

In a mixing bowl, combine the drained squash and onions with shredded sharp cheddar cheese, mayonnaise, beaten eggs, salt, pepper, and garlic powder. Mix in milk until well combined.

Transfer the mixture into your prepared baking dish and spread evenly.

In a separate bowl, crush buttery crackers and mix with melted butter. Sprinkle over the casserole.

Bake in the preheated oven for 35 to 40 minutes until bubbly and the topping is golden brown.

Let cool for a few minutes before serving.