Final answer:
The subordinate conjunction in the given sentence is 'once'.
Step-by-step explanation:
A subordinate conjunction is a type of conjunction that connects a subordinate (dependent) clause to a main (independent) clause. Subordinate conjunctions introduce adverbial clauses, which function as adverbs, providing additional information about the action in the main clause. These conjunctions help establish relationships of time, cause and effect, contrast, or condition between the two clauses.
The subordinate conjunction in the given sentence is 'once'.