6 tablespoons unsalted butter, divided
3 pounds (5 medium) yellow squash, sliced 1/4 inch thick
1 medium yellow onion, chopped (about 1-1/2 cups)
2 teaspoons kosher salt, divided
2 large eggs, beaten
1 (8-ounce) container sour cream
4 ounces sharp Cheddar cheese, shredded (about 1 cup)
2 ounces Swiss cheese, shredded (about 1/2 cup)
1/2 cup mayonnaise
2 teaspoons chopped fresh thyme
1/2 teaspoon black pepper
2 sleeves round buttery crackers (such as Ritz), coarsely crushed
1 ounce Parmesan cheese, shredded (about 1/4 cup)
Preheat oven to 350 degress. Melt 3 tablespoons of the butter in a large skillet over medium-high heat. 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. Transfer mixture to a colander set over a bowl. Drain 5 minutes; discard any liquid.
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. Spoon into a lightly greased 11- x 7-inch (2 quart) baking dish.
Microwave remaining 3 tablespoons butter in a medium-size microwavable bowl until melted. Toss together crackers, Parmesan cheese and melted butter until combined; sprinkle over casserole. Bake in preheated oven until golden brown, about 20 minutes. Serves 8.