Preheat your oven to 350°F (175°C).
Melt a tablespoon of butter in a large skillet over medium heat and add the chopped onions.
Cook the onions, stirring occasionally until they are golden brown, about 15-20 minutes.
In a mixing bowl, combine the cooled onions, sour cream, and cream cheese until smooth.
Fold in the shredded cheddar and grated Parmesan cheese, Worcestershire sauce, garlic powder, salt, and pepper.
Transfer the mixture into a baking dish and spread it evenly.
Bake in the preheated oven for 25-30 minutes or until bubbly and golden on top.
Remove from the oven and let it cool slightly before serving.