Preheat oven to 400 degrees F (200 degrees C). Lightly grease a 9x9 inch baking dish. In a bowl, whip together the egg yolks and heavy cream.
Melt the butter in a large saucepan over medium heat. Saute the onion and celery about 5 minutes. Season with salt, cayenne pepper, garlic powder, and Creole seasoning. Reduce heat to low, cover, and simmer 20 minutes, stirring occasionally, until very tender.
Mix the flour into the saucepan, and cook and stir continuously for 5 minutes. Mix in the egg yolk mixture. Stir in the 1/2 cup Cheddar cheese and American cheese until melted. Remove from heat and fold in the crabmeat. Transfer the mixture to the prepared baking dish.
Bake 20 minutes in the preheated oven, until bubbly and lightly browned. Remove from heat and top with the sharp Cheddar cheese and green onions. Allow the cheese to melt before serving.