With que, introducing a full clause in the perfect aspect.
Une fois qu'elle l'aura vu, elle le comprendra. ― Once she sees it, she'll understand it.
Introducing a reduced passive-voice clause consisting only of a subject plus a past participle phrase.
Une fois le déjeuner fini, vous pourrez remonter. ― Once dinner is finished, you can go up.
Introducing a reduced perfect-aspect clause consisting only of a subject plus a past participle phrase (provided that the verb is one that forms the perfect aspect with être).
Une fois les professeurs arrivés, l'élèves expliquèrent tout. ― Once the teachers had arrived, the students explained everything.
Introducing a reduced clause in either of the above senses, consisting only a past participle phrase, its subject being implied.
Une fois sortie de cette maison, elle se senta libéré.
Once she was out of that house, she felt liberated.