- 1 head cauliflower, cut into florets and thick stems discarded
- 3/4 cup walnut halves or pieces
- 1 (2-ounce) tin oil-packed anchovy fillets
- 5 cloves garlic, peeled
- 2 cups packed baby arugula
- 1 cup packed fresh flat-leaf parsley leaves
- 1/2 cup extra-virgin olive oil
- 3/4 teaspoon crushed red pepper
- 1/4 cup grated Parmigiano-Reggiano cheese, plus more for serving
- Finely grated zest of 1 lemon
- 1 pound spaghetti
- Salt and freshly ground black pepper
- Toasted Breadcrumbs, for serving 
Pulse the cauliflower florets in a food processor until they resemble coarse crumbs, with some pieces about the size of a pea or slightly smaller. Transfer to a bowl and set aside.
To the food processor (no need to clean out the bowl), add the walnuts, anchovy fillets with their oil, garlic, arugula, parsley, 1/4 cup of the oil, and the red pepper. Pulse until the mixture comes together to form a coarse paste. Add the cheese and lemon zest, and pulse briefly to combine. Set aside.
Bring a large pot of water to a boil and season generously with salt -- it should taste like seawater. Add the spaghetti and cook until it is al dente, usually 8 to 9 minutes. Drain the pasta, reserving 2 cups of the pasta water.
While the pasta is cooking, heat a 14-inch skillet over high heat with the remaining 1/4 cup oil. When the oil is very hot, add the cauliflower and cook, stirring frequently, until it begins to brown around the edges and smells nutty, 5 to 6 minutes. Stir in the arugula pesto and cook briefly, then remove the pan from the heat.
Return the skillet to medium-high heat and add the drained pasta and 1 cup of the reserved pasta water. Using tongs, toss the pasta with the cauliflower and pesto, adding more of the pasta water as necessary to form a sauce that just coats the pasta. Season with salt and pepper to taste, then serve immediately in pasta bowls, garnished with the Toasted Breadcrumbs and additional cheese.
A whole tin of anchovies? Believe me, it's not too much; in fact, I've been known to add more, as with the garlic. Make the recipe once as written, then tweak it to suit your own particular tastes.
The starchy pasta water is a key ingredient, acting as a thickener for the sauce.