"Once" is a subordinating conjunction here in the sense "when" or " as soon as". Like any other adverbial conjunction "once" can also locate the action of the main clause in the time frame being subservient but lending meaning to the main clause. ' Once ' may precede an adverbial clause or phrase.
Without "it is" the first part of the sentence is a reduced adverbial clause, a phrase. All 0/-ed/-en forms with out tense conjugation are passively adjectival. You're at liberty to use the reduced clause or its unabridged form. It's a matter of preference, nothing more to it.