Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish with vegetable oil or butter.

In a large skillet over medium heat, add 2 tablespoons of vegetable oil, sliced yellow squash, and Vidalia onion, sauté for 5-7 minutes until fork-tender.

Remove from heat, stir in 2 tablespoons of butter until melted, then fold in 1 cup of sour cream, 1 cup of shredded sharp cheddar, and half of the grated Parmesan cheese.

Pour the cheesy squash mixture into the greased baking dish, spreading it evenly.

In a separate bowl, combine the remaining grated Parmesan with crushed buttery crackers, then sprinkle over the casserole.

Bake uncovered for 25-30 minutes until the topping is golden brown and edges are bubbling.

Allow to cool for 5-10 minutes before serving.