Despite traveling to Vietnam a half dozen times to write about the cuisine, until now I had overlooked phở as a subject, perhaps because of its ubiquity on every Vietnamese restaurant menu. But one day while enjoying a bowl of it in New York City, I thought: Phở is the national dish of Vietnam, devoured from Ha Long Bay to the Mekong Delta. But where did it actually originate?

I knew its roots were in northern Vietnam. I also knew that phở bắc—northern phở—is different from the phở I’d been eating most of my life. In the U.S., we’re usually served the southern Vietnamese version, whose broth is sweeter and more herbaceous, and the dish comes with a side of sprouts, Thai basil, cilantro, hoisin sauce, sriracha, and jalapeño. In the north, the broth lacks the same sweetness, and the side accompaniments are usually pickled garlic and sliced red hot peppers. I’d fallen hard for northern-style phở.

When I asked friends in Vietnam about its origins, their responses varied. Some said phở—most easily pronounced fuh for English speakers—was first created in Hanoi. Others pointed to an outlying province. To complicate matters, theories differed about the influence of French colonialism and Chinese occupation. To trace the soup’s roots, not to mention find the best bowl of phở in Vietnam, I decided to make a pilgrimage to the capital, Hanoi.