Melt butter in pan. Add garlic and brown. In a large pan, add onion, lemongrass, garlic, sherry, salt, pepper, sesame oil and black bean sauce. Saute’ lightly. Add 3/4 cup of water and bring to a boil. Add washed clams to the pot. Mix well and cover. Bring to a boil until clams open. Sprinkle green onions on top and cover for 5 minutes.Take off stovetop and serve immediately.