Hun skreller ikke grønnsakene før hun har lest oppskriften én gang til.

In main clauses (V2 word order), Norwegian typically places the finite verb in the 2nd position and ikke after the verb:

• Hun skreller ikke grønnsakene.
  Putting ikke later often sounds unnatural or can change the focus.

Because after før you get a subordinate clause, and that clause needs its own subject. Norwegian normally repeats the subject:

• … før hun har lest …
  You can’t usually omit it the way English sometimes can.

Før introduces a subordinate clause, and subordinate clauses usually have the “subordinate” word order, where negation and many adverbs come before the main verb (but after auxiliaries):

• … før hun har lest oppskriften …
  If you had negation inside that clause: før hun ikke har lest … (though that specific meaning is odd here).

Because the sentence describes an action that must be completed before another action happens. Har lest emphasizes completion:

• før hun har lest oppskriften = before she has read the recipe (i.e., finished reading it)

Yes. Norwegian commonly uses:

• main clause in present: Hun skreller ikke …
• subordinate “before” clause with present perfect: før hun har lest …
  This neatly encodes “don’t do X until Y is done.”

Norwegian often uses the definite form when referring to specific, known items in context:

• grønnsakene = the vegetables (the ones involved here)
• oppskriften = the recipe (the relevant one)
  If it were non-specific, you might see indefinite forms like grønnsaker / en oppskrift, but the meaning would change.

-ene is the definite plural ending for many masculine/feminine nouns:

• en grønnsak (a vegetable)
• grønnsaker (vegetables)
• grønnsakene (the vegetables)

Both exist, but én (with the accent) strongly highlights the number one—one more time (exactly once more).
Without the accent, en can look like the article en (a), so én helps clarity and emphasis:

• én gang til = one more time

X gang(er) til means “X more time(s)” / “again (X times).”
It often comes at the end, but it can sometimes be moved for focus:

• … har lest oppskriften én gang til. (most neutral)

Skreller is present tense. Depending on context, it can mean:

• a current action: “is peeling”
• a habitual/general action: “peels”
  Here it’s typically understood as “she doesn’t peel (them) before …” (a rule/sequence).

That word order is generally not idiomatic in Norwegian. In a neutral main clause, ikke usually comes right after the finite verb:

• correct/neutral: Hun skreller ikke grønnsakene … Other placements tend to be marked or wrong in ordinary prose.

Yes, Norwegian før can also be used in time expressions meaning “ago,” but that’s a different construction (often with for or set phrases).
In this sentence, før clearly means “before/until” introducing a subordinate clause:

• … før hun har lest …