Consider the misunderstood schmaltz.

Chicken fat turns into schmaltz when gently rendered with a little water, then seasoned with onion. It’s a substance that is at once irresistible and scorned, decadent and sacred, formerly ubiquitous, now largely hidden and forbidden and so efficiently eradicated from culinary lexicon that its true definition is absent from leading dictionaries.

Then along came Michael Ruhlman, the James Beard Award-winning food historian, cookbook author and trained cook. He is America’s scholar on most things fat.

A great defender of humble, natural fats, he’s the technique-driven writer who taught us how to dry-cure meats in his book on salumi, and how to make sausage in his book on charcuterie.

Now he turns his attention to the centerpiece ingredient in traditional Jewish cuisine. He has written “The Book of Schmaltz: Love Song to a Forgotten Fat” (Little Brown, $25).

Ruhlman, who is not Jewish, came to the topic as an outsider. He saw his opportunity to explore his interest in schmaltz about two years ago during a conversation at a neighborhood party.

It was just before Rosh Hashana and Yom Kippur and one of his neighbors, an accomplished home cook named Lois Baron, announced that in preparation for the High Holy Days, she was off to make schmaltz. “There’s nothing else like it,” said the 78-year-old neighbor who, as Ruhlman writes, “cooks like a banshee.”

The cookbook author asked his neighbor if she would join him on a chicken fat adventure, a book that would serve as a tutorial and reference guide on schmaltz. After a good laugh, she agreed.

“She became my gateway to the world of schmaltz,” Ruhlman said by phone last week from his Cleveland home.

They worked together and self-published their schmaltz journey as a digital app. Ruhlman’s wife, photographer Donna Turner Ruhlman, documented the lush, fat-crisped creations for the project. When the author showed the product to his editors at Little Brown, they snapped it up.

Born out of poverty

Ruhlman says he was smitten from the first dish he composed with the velvety goop.

“The first thing I cooked with it was chopped liver, and it was a revelation,” he said. “Here was another denigrated food — as in ‘What am I, chopped liver?’ — something so ordinary, born out of poverty. But if you get good chicken livers, they’re like nothing else, and they’re so good.”

So good, but so feared and maligned.

“People have a knee-jerk reaction when you say the word schmaltz — especially Jews. ‘Schmaltz? Give me a heart attack right now!’ But you’re not eating gallons of it. You’re using it for flavor. It’s more healthy than butter if you use it instead of butter,” said Ruhlman.

Cooking with schmaltz is a little like cooking with duck fat, the now ever-present ingredient at foodie haunts and gastro pubs. “But there’s less of it. Chickens have less fat relative to ducks. Schmaltz is harder to get,” he said.

But it’s worth the effort. “Just cooking potatoes in schmaltz gives them an incredibly crispy finish,” Ruhlman said.

An easy process

To achieve about a half-cup of good schmaltz, he buys a pack of eight bone-in chicken thighs — he also reserves the fat when he cooks other chicken dishes. He renders the chicken thigh fat with an onion to prepare the schmaltz. The prepared fat keeps in the refrigerator for about five days — more than that and the flavor becomes muted. Or you can freeze it for six months.

“It’s not that tricky to make. It’s just fat with a little water — the water will cook off. Render the fat and once it’s out of the skin, you add the onion and it will cool it down and slowly flavor the fat. This will produce the gribenes [crispy bits]. Then you just strain it,” he said. It’s been done this way for centuries.

“I’m fascinated by Jewish culture and how food cultures in general evolve and why. This food culture was born out of poverty, in poor villages in Eastern Europe, using liver and inexpensive cereals, wheat, oats, groats, buckwheat,” he said.

“I hope people appreciate schmaltz for what it is. It’s tradition,” he said. “And the flavor is wonderful.”