Preheat oven to 350 degrees Fahrenheit. Lightly butter an 8×11 baking dish or casserole. Set aside.
In a large skillet, melt the butter over medium high heat. Stir in the sliced squash and onions. Sprinkle with salt and pepper. Cook until the squash are tender, stirring occasionally, about 10 – 15 minutes.
Remove from heat and fold in shredded cheddar cheese, parmesan, sour cream and mayonnaise.
Transfer to the buttered casserole dish. Sprinkle the top evenly with crushed Ritz crackers.
Bake uncovered for 25 minutes until the top is golden and bubbly.