Preheat oven to 400°. Lightly grease a 9x9 baking dish
Whip together egg yolks and heavy cream.
Melt butter in saucepan over medium heat. Sauté onion and celery for 5 minutes. Season with garlic powder, cayenne pepper, salt, and Creole seasoning. Reduce heat to low. Cover, and simmer 20 minutes. Stirr occasionally until tender.
Mix flour into saucepan. Stir continuously for 5 minutes.
Mix in the egg yolk mixture. Stir in the ½ cup Cheddar cheese and American cheese until melted.
Remove from heat and fold in crabmeat. Transfer mixture to prepared baking dish.
Bake 20 minutes in preheated oven. Result will be bubbly and lightly browned.
Remove from heat and top with sharp Cheddar and green onions. Serve when Cheddar has melted.