She rumbles down a dirt path in a remote corner of Mexico in the Nissan pickup she’s put many hard miles on. The road’s so bumpy her ever-present straw hat drops over her eyes, not that this stops her.

Diana Kennedy doesn’t bother to adjust it in front of the camera operator. She pumps the clutch, shifts to a lower gear and waits for the next bump to give her back her clear view of the rutted road ahead.

She doesn’t look it or act it, but she’s well over 90. You’d never think it, but this 90something British expat is the doyenne of Mexican cuisine, a towering figure in the popularization of the “authentic” foods that have spread worldwide, and all but taken over the American palate. And you’d never guess it, but this tiny dynamo is still scouring her adoptive land for new flavors, new foods, new recipes.

“Diana Kennedy: Nothing Fancy” is an adorable and adoring portrait of the outspoken foodie, chef, cooking teacher and cookbook author whose 1972 book “Cuisine of Mexico” put salsa and ceviche, Pozole and mole on the culinary map and on the road to global popularity.

First-time documentarian Elizabeth Carroll lets the TV-seasoned Kennedy, whose cooking-on-the-tele days are decades behind her, just be her charming, blunt and bawdy self for a film portrait that is the next best thing to being declared a walking, talking, cursing and cooking U.N. Heritage Site.

If your first thought on hearing of an Englishwoman’s role in making Mexican cuisine popular is “cultural appropriation,” you might be surprised how that came up with her publishers, way back in 1970. She’s keenly aware of it, still.

“An Englishwoman making GUACAMOLE,” she fusses and chuckles. What matters is making it authentic, she huffs. “Do it RIGHT,” the way they do in the provinces. “No, you don’t put garlic in. You don’t use kosher salt!”

Use a mortar and pestle, “not a food processor.” And Madre de DIOS don’t turn it into a cream. “Leave it LUMPY!”

Carroll sits in on a cooking class where just that sound of brutally frank instruction is what well-heeled cooks and restaurateurs can expect. “I have five restaurants in Portland” and “I have three in Manhattan” may carry water there. Not with Kennedy, and not if you’re doing it wrong.

“You never stir the rice. NEVER.” “I cannot BEAR this ‘salt-less cooking,'” she says, and that’s before she corrects some hapless cook who has dared sub in garlic when SALT is what is called for.

José Andrés calls her “an Indiana Jones of food,” thanks to her expeditions (sampled here) in search of this or that. Mexican chef and TV show hostess Pati Jinich calls her “a prophet for Mexican food.”

“If her enthusiasm were not beautiful, it would border on mania,” longtime New York Times food editor Craig Claiborne, an early champion, wrote in the foreword to that epochal first cookbook (she’s written eight).

She smiles and stalks through village markets, enthusing for traditional dishes prepared “traditional ways,” sneering at a stall where the cook has used “food coloring.”

Carroll’s film takes Kennedy and us through her life story, “propelled by lots and lots of hormones” to be a world traveler, meeting her future husband, New York Times Caribbean correspondent Paul Kennedy in Haiti in the middle of a revolution.

Her enthusiasm for the food at their Mexican stop on Kennedy’s career-travels may be surprising. She had Mexican cookbooks to consult (by Josina Velasquez de Leon), but preferred exploring and finding recipes on her own. Cooking for visiting journalists and Times editors were her ticket to fame, recreating classic Mexican dishes for New York naifs, guided into publishing by a friendly ear, Times editor Claiborne.

As she fusses that “There’s so much more that I’d love to do,” about others “plagiarizing my recipes,” as she jokes “Thank God my black panties don’t show” at a photo shoot, you can’t help but fall for this twinkling spitfire, this Earth Mother (she’s big on “organic” ingredients and green landscapes) of Michoacan.

And as she tastes the foods of the country, delighting in this old favorite or that new regional wrinkle on a traditional recipe, you may find yourself fretting that you’re watching this on an empty stomach.

MPAA Rating: unrated, profanity

Cast: Diana Kennedy, José Andrés, Gabriela Camara, Alice Waters, Abigail Mendoza

Credits: Directed by Elizabeth Carroll. A Greenwich Entertainment release.