Preheat oven to 350 degrees. Butter a 9 x 13 baking dish and set aside.
In a large skillet melt 4 TBS butter over medium high heat. Cut squash into 1/4 - 1/2 inch rounds and place into skillet. Add in chopped onion. Season with a 1/4 tsp each of salt, black pepper, garlic powder, and onion powder.
Cook squash and onion for 6-7 minutes until partially softened. They will cook the rest of the way once in the oven later in recipe. Turn off heat.
In a large mixing bowl whisk together beaten eggs, sour cream, mayonnaise, 1/2 cup cheddar cheese, and 1/2 cup swiss cheese. Season with 1/2 tsp each salt, black pepper, garlic powder, and onion powder.
Fold in cooked squash and onion into the sour cream/mayonnaise mixture until evenly coated. Pour mixture into prepared baking dish evenly. Top with remaining 1/2 cup cheddar cheese and 1/2 cup swiss cheese.
In a small bowl mix together melted butter and ritz cracker crumbs until well coated. Sprinkle mixture all over casserole in an even layer.
Bake in preheated oven for 20-25 minutes until cracker topping is golden brown and cheese has melted. Let cool for 5 minutes before serving.