Preheat oven to 350 degrees and lightly grease a casserole dish.
Steam squash and onion until just tender, about 5 to 7 minutes.
Season squash and onion with salt and pepper.
In a large bowl, whisk together egg, mayonnaise, sour cream, and milk.
Stir in cheese, squash and onion. Transfer mixture to casserole dish.
Crush crackers and scatter on top of squash. Drizzle with melted butter.
Bake for 30 minutes.