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My wife, Carrie, and I decided to stop at Grapeseed American Bistro in Bethesda on Friday after a late-night workout. We choose two of the counter seats next to the open kitchen, a vantage point I love even if it has the effect of turning hard-working line cooks into zoo animals.

Not wanting to overload on calories after exercise, we split a pair of appetizers and a single entree, a crispy chicken breast paired with a side dish that Chef Jeff Heineman calls a “shepherd’s pie.” It’s more like a loose mold of dark chicken meat, black beans, and peppers, topped with a piped Dairy Queen–like swirl of mashed potatoes. It was the spuds that caught our attention. They were white…and sweet. Carrie and I immediately launched into a guessing game. She figured the mashed taters were spiked with sugar. I suggested a honey-sweetened butter might have done the trick.

We were both wrong.

Turns out Heineman uses boniato potatoes. I was embarrassed to admit that I had never sampled them before. According to the Institute of Food and Agricultural Sciences at the University of Florida, “Bonatios are also commonly called Cuban sweet potatoes. While the generic name is the same as for the ordinary sweet potato widely grown in gardens and farms around the state, boniatos differ primarily by having a distinctive white interior rather than the characteristic yellow or orange flesh of other varieties.”

The potatoes, says author James M. Stephens, professor in the Horticultural Sciences Department with IFAS, “have been grown throughout the subtropical world for centuries, but became an important commercial crop in Florida in recent years. Popularity rose with the increasing population of Cubans in the Dade County area. Since there was always a small Cuban-Latin sector in South Florida, boniatos were grown for home use and limited sales for many years before that.”

Stephens notes that “interest in [boniatos] outside Dade County is limited.” Limited to a small restaurant in Bethesda, as far as I know. Have you seen them elsewhere?