Melt butter in a deep saucepan over medium heat. Add onion, thyme and pepper; cook until golden.
Stir in chicken broth and beer. Bring to a boil; simmer over low heat for 10 minutes.
Whisk egg yolks until pale and thick. Whisk in cream. Stir some of the hot soup mixture into the cream mixture. Stirring constantly, gradually add the cream mixture to the soup. Toss cheese with corn starch; stir into soup.
Cook for 5 minutes; stirring until cheese is melted, but not allowing the soup to boil.
Strain the soup through a sieve. Stir in lemon juice and hot sauce to taste. Ladle into soup bowls.