Melt the butter in a large skillet over medium heat. Add thinly sliced Vidalia onions, stirring occasionally for about 15-20 minutes until they are golden brown and caramelized.

In a mixing bowl, combine cream cheese and sour cream. Blend until smooth and creamy, then add half of the shredded cheese blend, garlic powder, salt, and pepper. Mix well.

Grease a 9x13-inch baking dish with butter. Spread half of the caramelized onions on the bottom, followed by half of the cheese mixture.

Top with the remaining caramelized onions and the rest of the cheese mixture. Sprinkle the remaining shredded cheese on top.

Sprinkle breadcrumbs evenly over the top and drizzle with melted butter.

Preheat the oven to 350°F (175°C). Bake for 30-35 minutes or until golden brown and bubbly.

Let the casserole cool for about 5 minutes, then garnish with chopped parsley and grated Parmesan cheese before serving.