Preheat oven to 350°F. Spray an 8-inch or 9-inch square baking dish with cooking spray and set aside.
Melt 1 tablespoon butter in a large skillet over medium heat. Add squash, onion, and bell pepper to the skillet, cooking and stirring for about 10 minutes (or until vegetables are tender). Remove from heat and season with salt and pepper.
Meanwhile, in a separate bowl, whisk together eggs, cheese, sour cream, and mayonnaise.
Stir the egg mixture into the squash mixture.
Transfer the squash mixture into the prepared baking dish.
In a separate bowl, combine crushed crackers with remaining 1 tablespoon of melted butter. Sprinkle buttered cracker crumbs over top of casserole.
Bake for 30-35 minutes, or until casserole is set and topping is golden brown.
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