It's all over a recent segment on "The Tonight Show" -- in which Jay pokes fun at the homes of several GOP candidates. We'll spare you, but the punchline is that Mitt Romney -- a famously wealthy businessman -- spends his summers in a solid gold palace.

As an illustration, Jay pulled up a photo of India's Harmandir Sahib -- aka the Golden Temple of Amritsar -- one of the most sacred buildings in the world to the Sikh people.

There's been a lot of outrage among Sikhs over the clip -- but one man in California named Dr. Randeep Dhillon took his religious fervor one step further ... filing a libel lawsuit against the late night host, claiming Leno is responsible for encouraging hatred and ridicule of his religion.

According to Randeep, the Romney joke "falsely portrays the holiest place in the Sikh religion as a vacation resort owned by a non-Sikh" ... THE HORROR.