Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish generously with butter or cooking spray.

Finely dice one onion and slice your yellow squash into 1/4-inch thick rounds.

In a large skillet, melt 2 tablespoons of unsalted butter over medium heat. Add the diced onions and sauté for about 2 minutes until translucent and fragrant.

Add the sliced yellow squash and cook for 2 minutes. Pour in a splash of water, cover the skillet, and steam for about 5 minutes until tender.

In a mixing bowl, combine one can of condensed cream of mushroom soup, 1 cup of sour cream, and a can of diced green chiles. Add 1 teaspoon of garlic powder and 1/2 teaspoon of ground black pepper, whisking until smooth.

Fold the sautéed squash and onions into the creamy mixture. Gently incorporate 2 cups of herb seasoned stuffing mix.

Transfer the mixture into the prepared baking dish, spreading it evenly. Top with shredded sharp cheddar cheese.

Bake in the preheated oven for 25 to 30 minutes until the cheese is golden brown and the edges are bubbling.

Allow the casserole to rest for 2-3 minutes before serving.