Preheat your oven to 350°F.
While it heats, lightly grease a 9×13-inch baking dish.
Cook the zucchini and onions until tender.
You can either sauté or boil them.
Afterward, drain any excess liquid.
In a large bowl, combine cheddar cheese, sour cream, mayonnaise, and the beaten egg with the cooked zucchini and onions.
Mix everything well until evenly distributed.
Spread this mixture evenly in the prepared baking dish.
Combine the melted butter with the crushed crackers in a separate small bowl.
Spread the cracker mixture evenly over the zucchini mixture in the baking dish.
Bake for 30 to 35 minutes, or until the top is golden brown and the casserole is heated through nicely.