How to Make It
Preheat oven to 350°. Place first 3 ingredients in a Dutch oven; add hot water 2 inches above squash-and-onion mixture. Add salt to Dutch oven, and stir to dissolve. Bring mixture to a boil over medium-high heat. Boil 10 minutes or until squash is very tender when pierced with a fork. Drain vegetable mixture well; let stand at room temperature 15 minutes to cool slightly.
Stir together sour cream and mayonnaise in a large bowl. Lightly beat egg, and stir into mayonnaise mixture. Gently stir in cheese, pepper, and cooked squash mixture; spoon mixture into a greased (with butter) 13- x 9-inch baking dish.
Bake at 350° for 40 minutes or until bubbly. Combine cheese crackers and melted butter, and toss to coat. Arrange over casserole. Bake 10 more minutes. Remove from oven, and let stand 5 minutes before serving.