If a Muslim misses Fajr prayer due to sleep and wakes up after sunrise, should he first pray the Fajr supererogatory prayer and then the obligatory prayer, or the reverse?

The first is to pray the Sunnah of Fajr first, then the obligatory prayer, following the example of the Prophet (peace and blessings be upon him) and his companions. Indeed, they missed Fajr during a journey, so they prayed the two rak'ahs (Sunnah) then the obligatory prayer after the sun had risen.