Slice your yellow onions into thin rings, about 1/4 inch thick.

Melt 4 tablespoons of butter in a large skillet over medium heat.

Add the sliced onions, salt, and pepper, then cook for 25 to 30 minutes, stirring occasionally, until they turn deep golden brown and become very soft.

While your onions cook, preheat your oven to 350 degrees F.

In a medium bowl, whisk together sour cream, milk, flour, and garlic powder until smooth.

Stir the shredded sharp cheddar cheese into the sour cream mixture until the cheese melts completely.

Pour the caramelized onions into a 9x13 inch baking dish. Add the cheese sauce and stir gently until all the onions are coated evenly.

Sprinkle 1 cup of fried onion strips over the onion and cheese mixture, stirring them in gently.

Bake uncovered for 20 minutes until the mixture is hot and bubbling at the edges.

Remove the dish from the oven and top with the remaining 1 cup of fried onion strips.

Return the casserole to the oven and bake for an additional 5 to 8 minutes until the fried onions on top turn golden brown and crispy.

Let the casserole rest for 5 minutes before serving.