What should one do who forgot to recite Al-Fatiha in the first Rak'ah and remembered it during the last Tashahhud?

The preponderant opinion is that Al-Fatiha is a pillar of prayer, without which the prayer is invalid, whether for the Imam, the congregant (Ma'mum), or the individual performing prayer alone. So, whoever forgets Al-Fatiha in the first Rak'ah and remembers it during the final Tashahhud, should perform an additional Rak'ah to compensate for the incomplete one and perform the prostration of forgetfulness (Sujud Al-Sahw). The congregant should perform it after the Imam concludes the prayer. If the congregant remembers forgetting it after bowing with the Imam, he should perform a Rak'ah after the Imam concludes the prayer. As for the majority of scholars, they hold that Al-Fatiha is waived for the congregant because the Imam bears the responsibility of its recitation on his behalf.