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'.

So here, 'earlier today' is a fronted adverbial. Here are some more examples. Calmly, the family sat together and watched a movie. Like a cheetah, Bill sprinted to the finish.