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

In a skillet, heat 2 tablespoons of vegetable oil over medium heat. Add the sliced yellow squash and Vidalia onion, cooking for 5-7 minutes until tender and fragrant.

Stir 2 tablespoons of melted butter into the cooked squash and onions. Transfer to a bowl and fold in 1 cup of sour cream, 1 cup of shredded sharp cheddar cheese, and 0.5 cup grated Parmesan cheese. Blend until combined.

Pour the creamy mixture into the greased baking dish and spread evenly across the bottom.

In a separate bowl, combine 1 cup of crushed buttery crackers with 0.5 cup grated Parmesan cheese. Mix well and sprinkle over the vegetable layer.

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

Let the casserole cool for 5-10 minutes before serving for easier slicing.