Heat olive oil in a large skillet, then add squash and onions.
Cook until the vegetables begin to soften. Drain excess liquid.
Place the mayonnaise, egg, cheddar cheese, Parmesan cheese, flour, garlic powder, salt, and pepper in a large bowl.
Stir the ingredients until everything is well combined.
Add the cooked squash and onions to the mayonnaise mixture.
Stir to combine ensuring the vegetables are well coated.
Place crushed crackers, Parmesan cheese, and melted butter in a small bowl.
Stir to combine and until the crackers are moistened.
Transfer the mayonnaise squash mixture to a 2.5 quart baking dish. Spread into an even layer.
Sprinkle the cracker mixture over the top of the mayonnaise squash mixture. Bake on 350°F for 30 minutes.
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