Peel a medium onion and cut it in half. Cut a thin slice from bottom and top so it sits flat. Scoop out a 1-inch-deep hole from the top center of each onion. Place onion on a microwave-safe dish with a lid. Add 1/2 the beef bouillon and 1/2 the butter to each shallow hole. Cover and microwave at HIGH for 4 to 5 minutes or until onion is tender. Garnish each serving with fresh parsley sprig and pepper, if desired.