Drain and pat dry oysters. In a skillet, saute onions in butter until translucent, about 5 minutes. Add oysters and cook until edges curl, about 30 seconds. Remove from heat.
In a bowl, whisk together eggs, beer, salt, garlic powder and hot sauce. Pour egg mixture over oysters and cook on low, stirring until eggs are almost set. Sprinkle shredded cheese on top and cook until eggs are set, about 15 minutes. Crumble bacon on top and add chopped parsley and diced tomatoes before serving. Season with salt, if desired.