Preheat your oven to 350°F (175°C) and grease a 9x13-inch casserole dish.

In a skillet over medium heat, heat vegetable oil and sauté thinly sliced yellow squash and diced Vidalia onion for 5 to 7 minutes.

Remove from heat, stir in butter until melted, then combine sour cream, shredded sharp cheddar, and half of the grated Parmesan cheese.

Pour the mixture into the casserole dish and spread evenly.

In a bowl, mix the crushed buttery crackers with the remaining Parmesan cheese and sprinkle over the casserole.

Bake uncovered for 25 to 30 minutes until the top is golden brown.

Let it cool slightly, then serve as a hearty side dish.