The Semi Slav- d4 d5 c4 c6 Nf3 Nf6 Nc3 e6 there are many main lines after this but the ones that most good players play are either e3 or Bg5. Personally I think that Bg5 is better. But e3 is played by many players too and it's not bad.

Here is a game that legendary Botvinik played against Kin with the Semi Slav where white played e3

here is a game were Bg5 is played instead of e3