Daren Metropoulos is fighting back in court against the assistant his company hired in 2014. According to docs, obtained by TMZ, the woman resigned in early 2017 ... and 3 months later started demanding millions. Metropoulos -- who's also heir to the Pabst Blue Ribbon fortune -- says she threatened to spread lies about him mistreating and harassing women.

He says the ex-assistant targeted him on the heels of his mega-deal to purchase the Mansion.

In the suit, Daren also says she lied like a rug during her interview -- failing to disclose she'd been sued for being an unlicensed contractor, or that she was once sued for fraud and fabricated portions of her resume.

As he puts it in the suit ... "She is an opportunist; a scammer; and an extortionist."

Daren's lawyer, Marty Singer, tells TMZ, "In nearly 40 years of practice, I have never seen such a blatant case of fraud and concealment committed by an employee on her resume and during the job interview. Ms. Graves lied to the Metropoulos Company regarding her background and experience and concealed charges of fraud made against her in prior lawsuits."

Singer goes on, "I have represented the Metropoulos family and their company for years. The Metropoulos Company has employed numerous women over the years, some of whom have worked for the Company for over 15 years. No current or former employee has ever claimed any form of harassment or wrongful conduct by Mr. Metropoulos."

Metropoulos is suing her for more than $1 million.