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An essential relative clause provides necessary, defining information about the noun. On the other hand, non‐ essential relative clauses provide additional, non‐necessary information about the noun.
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'.
A fronted adverbial is when the adverbial word or phrase is moved to the front of the sentence, before the verb. Earlier today, I discovered fronted adverbials.