Ingredients

1. Prepare oven and baking dish: 

   Preheat oven to 350°F. Grease an 11- x 7-inch (2 quart) baking dish with cooking spray.

2. Cook squash: 

   Melt 3 tablespoons of the butter in a large skillet over medium-high. Add squash, onion, and 1 teaspoon of the salt; cook, stirring often, until center of squash is just tender and liquid has evaporated, about 10 minutes.

3. Drain liquid: 

   Transfer mixture to a colander set over a bowl. Drain 5 minutes; discard any liquid.

4. Stir together casserole filling: 

   Stir together eggs, sour cream, Cheddar and Swiss cheeses, mayonnaise, thyme, pepper, and remaining 1 teaspoon salt in a large bowl. Gently fold in squash mixture.

5. Spoon casserole filling into baking dish: 

   Spoon into a lightly greased 11- x 7-inch (2-quart) baking dish.

6. Make cracker topping: 

   Microwave remaining 3 tablespoons butter in a medium-size microwavable bowl on HIGH until melted, about 25 seconds. Toss together crackers, Parmesan cheese, and melted butter until combined.

7. Sprinkle topping over casserole: 

8. Bake and serve: 

   Bake in preheated oven until golden brown, about 20 minutes.