A broccoli sauce is a hard sell anywhere in the world, but particularly in Rome, city of—let's be honest—more alluringly simple pastas like cacio e pepe and carbonara.

So I expected little from the primo at Pipero Roma—Allesandro Pipero's posh restaurant not far from the Pantheon—rigatoni dressed sparingly with a thin, bright green sauce I initially mistook as pesto.

One bite dismissed that notion, but also didn't provide clarity. The pasta, of course, was perfectly al dente. The sauce—most definitely not pesto—was light, tasting as bright as its hue—thin, clean and herbal, but still rich.

In fact, it tasted little of broccoli. Not sulfurous or harsh. Nor raw or fibrous. Just simple, smooth freshness with rich, savory flavors lingering behind it.

It was good enough to draw me into the kitchen, where sous chef David Puleio explained how they extract such delicate flavor from so robust a vegetable. The answer was entirely unexpected—broccoli leaves.

In fact, the dish is one of a family of Italian recipes in which a boldly flavored green vegetable—asparagus is another common choice—is blanched until tender, then pureed with simple seasonings and a bit of starchy pasta cooking water.

The result is a light, bright sauce that belies its starting point. The result—and intent—is to enhance rather than cloak the pasta.

Pipero Roma's version was a simple emulsion of blanched broccoli leaves pureed with butter—an unusual choice in olive oil country—garlic, red chili and a splash of water. It came together in minutes, the finishing flourish a bit of the city's go-to pecorino Romano cheese.

Simple though it was, bringing this back to Milk Street brought challenges. Broccoli leaves are few on the crowns sold at U.S. supermarkets. An alternative clearly was needed.

We considered an entirely new green—spinach—but that was too grassy. Instead, we worked with what was available and abundant—the broccoli stalks. Peeled of their tough skins, they pureed wonderfully—developing the same silky, smooth texture I'd had in Rome—after a brief boil.

We kept the garlic and butter—which gave the sauce a creamier note than olive oil—but favored the ease and convenience of red pepper flakes over a fresh chili. But while the flavor was good, the color was wane, nothing like the vibrant green I'd loved in Rome.

The solution brought us back to spinach. Just a bit of baby spinach added to the broccoli brightened the dish, keeping both the flavor and color bright. We also found the sauce benefited from stirring a bit of the cheese into it, giving it a richer body and deeper savory flavor.