Assalamu aalaykom wa rahmatullahi wa barakatu. I read the reconciliation from Ibn 'Abbas Radiallahu 'Anhu. He said Allah created the Earth, then created the heavens (made them into 7) then he spread out the earth and added the water and the mountains etc. But, in Surah Fussilat it actually says the opposite. Everything was added on the Earth AND THEN Allah went to the Heavens to fashion them. So can you further clarify for me Inshallah? Maybe I missed something. Jazakullahu Khairan!

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

What you read is correct, and it is the way to reconciliation between the verses that seem to be contradictory, for Allah the Almighty has told in Al-Naziaat that He spread the earth, that is, He created its sustenance and what is on it after creating the Heaven. He says (what means): {And after that, He spread the earth, and brought forth therefrom its water and its pasture.} [Quran 79: 30-31]

What is in Surah Fussilat in the verse, “...And He blessed it and determined therein its [creatures'] sustenance in four days...”, it means in the duration of four days, that is, in two days added to the first two days, and these two days are after the two days of the creation of the sky. The conjunction here in Al-Fussilat is not for order, as evidenced by the verse of An-Nazi’aat. Al-Shanqeeti discussed this in detail in his book Daf' Ihaam al-Ittirab 'an Ayy al-Kitab, so you can refer to it if you like.

Allah knows best.