Wash hands with soap and water.
Preheat the oven to 350°F.
Scrub the onion and squash with a clean vegetable brush under running water.
Slice the onion and squash.
Heat oil over medium heat in a large skillet. Add the squash, onion and butter to the skillet..
Cook onions and squash until they are soft, about 5 minutes.
Transfer to a bowl and stir in Parmesan cheese, cheddar cheese, sour cream, salt and pepper.
Place mixture into a greased casserole dish and sprinkle the cracker crumbs evenly over the top.
Place the casserole dish on the center rack in the preheated oven and bake until the top is golden and bubbly, about 20 minutes.
Serve immediately.