Melt butter in a skillet over medium heat. Add the onion, garlic and thyme. Cook, stirring constantly, until onion is tender. Stir in the beef broth, scraping any onion bits from the bottom of the pan, then stir in the port wine. Bring to a boil, and cook until the mixture has reduced to about 1/2 cup. Set aside. This may also be made ahead of time, and reheated.