Which chapter of the One Punch Man webcomic should I start on after seeing the anime? I just finished episode 12 of the anime and I'm under the impression that this is the end of season 1. I'd like to continue with the manga now. I don't want to read the remake; I want to read the original webcomic that started in 2009.

Which chapter should I start on?
You should start from Chapter 42 of the original manga.

After the destruction caused by Boros and his defeat by Saitama, the space ship is taken by Metal Knight. The Hero Association HQ is rebuilt.

Chapter 42 picks off with S Class hero, King (ranked 7) facing off a reptile monster and then, Dushimof, a machine god created by the Organisation.