Preheat your oven to 350°F (175°C).
In a large skillet, heat the olive oil over medium heat. Add the chopped onion and sauté until translucent, about 5 minutes. Add the sliced squash and cook for an additional 5-7 minutes until tender. Remove from heat and let cool slightly.
In a large mixing bowl, combine the milk, eggs, mayonnaise, salt, black pepper, garlic powder, and onion powder. Whisk until well blended.
Stir in the sautéed squash and onion mixture, followed by the shredded cheddar cheese and crushed crackers. Mix until everything is evenly coated.
Pour the mixture into a greased 9x13-inch baking dish. Sprinkle the grated Parmesan cheese evenly over the top.
Bake in the preheated oven for 30-35 minutes or until the top is golden brown and the casserole is set.
Let it cool for about 5 minutes before serving.