Chef Gordon Ramsay -- If I'm Paying, YOU're Paying ... Goes After Fat Cow Partner

If Gordon Ramsay is going down, his ex-business partner's going with him -- the celeb chef is demanding some cash to cover big losses on his failed Fat Cow restaurant.

Ramsay filed court docs ... saying when he and Rowen Seibel opened the restaurant together in 2012, they agreed to split costs 50-50, if there were any lawsuits over the lease.

As we reported ... Ramsay IS being sued -- for at least $6 million. The landlord claims he bailed on a long-term lease when he shut down Fat Cow earlier this year.

So, now Gordon wants a judge to force Seibel to pay his share of $300K in legal fees ... and, if necessary, cough up half of the money if he loses the landlord's suit. We reached out to Seibel ... so far, no word back.