Preheat oven to 350º F.

Melt 2 tablespoons butter in medium skillet over medium-low heat. Add squash and onions and cook until tender.

In a large bowl, add eggs and lightly whisk. Add cheese and milk and whisk into egg until well-combined. Add cooked squash and onions to the egg mixture and stir well to combine. Melt remaining 2 tablespoons butter in the skillet used to cook squash and onions and add to the mixture. Stir in cayenne pepper, if using, along with salt and pepper. Stir well to combine.

Spray a 9x13 casserole dish with cooking spray and pour squash casserole mixture into the dish. Top evenly with crushed Ritz crackers. Bake for 45 minutes, or until the top has lightly browned and casserole does not jiggle when moved.

Allow to sit for about 3 minutes before serving.