In a large pot on medium-high heat, sauté the garlic and onion in the oil, stirring for 30 seconds. Add the clams and cook, stirring for 1 minute.
Add the sherry and wine and bring it to a boil. Add the Gordal olives and parsley, cover and cook for around 6 to 8 minutes or until the clams open. Serve sprinkled with Aleppo pepper, along with crusty bread.