Praise be to Allah.
If you are praying Fajr prayer before the sun rises, then the Sunnah in your case is to pray the regular Sunnah of Fajr which is two rak’ahs. If you miss Fajr and do not wake up until after the sun has risen, then you should pray Sunnah and then Fard (obligatory prayer). But if you always delay Fajr prayer until after the sun has risen, this is a serious problem. In this case, before you ask about the Sunnah prayer you have to pause and ask yourself what you will say when you stand before Allaah on the Day of Resurrection, and He calls you to account for neglecting Fajr prayer.
See also question no. 266.