A relative clause can be introduced by either a relative pronoun or a relative adverb. A relative clause can be restrictive or nonrestrictive (essential or nonessential) Restrictive relative clauses ...

Adverbs are a class of words that express time, location or manner, or add modification to whole clauses. “Unfortunately, they were out of ice cream.” “I’ll see you tomorrow.” ...

In this case, ‘Not until I saw what happened’ is the adverbial clause. The inversion takes place after this, in the main clause. This happens with 'Not +time' and 'Only + time'. 'Not since ...