The verb here is commands: to give an order or direction in an authoritative manner. MW, Oxford.
The way to tell which word is the verb is by analyzing the sentence's structure and grammar. Look up the words in question; which ones are possible verbs? Here, only commands and mock. Which of those two makes sense as the verb? It must be commands, because:
- Mock is not conjugated. The subject is Young, which is third person singular (as it's the name of a person), so Young commands is in the correct form, but Young mock is not.
- The sentence structure dictates it. If mock is the verb, the grammar makes no sense. Commands as a noun has no meaning at the start of the sentence here. In order to make this work, the sentence must be changed or assumed to contain significant errors. Conversely, if commands is the verb, the sentence does not require editing.
- The context gives us clues. "Dialogue" [speaking verb] [subject] and "Dialogue" [subject] [speaking verb] are the standard constructions (along with putting the quoted dialogue after rather than before). This gives us a hint that commands is the verb, but does not guarantee it.
Note that I'm assuming the sentence is written as intended. It's correct English as it stands, but a few seemingly minor changes could alter everything.
So what is mock? Here, it's an adverb modifying imperiously. Young's imperiousness is phony, sarcastic or teasing; it's not genuine. I would have written this as mock-imperiously to make it more clear, but it's not wrong.