Steam the cauliflower florets and core over boiling water for about 3 minutes. Taste a piece. It should be on the verge of tenderness and not quite fully cooked. Set it aside.
Bring a large pot of water to a boil for the pasta.
Heat the oil in a wide skillet over medium heat. Add the onion and saffron and cook, stirring frequently, until the onion is soft, 6 minutes or so. The steam will activate the saffron so that it stains and flavors the onion. Add the garlic, pepper flakes, and a few pinches of the parsley, give them a stir, and then add the cauliflower. Toss the cauliflower to coat it with the seasonings, add 1/2 cup water, and cook over medium heat until the cauliflower is tender, just a few minutes. Season with salt, toss with half of the remaining parsley, and keep warm.
While the cauliflower is cooking, cook the pasta in the boiling water seasoned with salt until al dente. Drain, transfer to a warmed bowl, and toss with a few tablespoons of oil and the remaining parsley. Taste for salt, then spoon the cauliflower over the pasta, wiggle some of it into the pasta crevices, grate the cheese on top, and serve.
With Shrimp: When wild Gulf shrimp are in season, take advantage of their sweet goodness. Peel 1 pound shrimp, then sauté them over high heat in olive oil until pink and firm, after 5 minutes or so. Toss them with chopped garlic and parsley and divide them among the individual pasta plates or heap them over the top of the communal dish. Omit the cheese.
Reprinted with permission from Vegetable Literacy by Deborah Madison, copyright © 2013. Published by Ten Speed Press, a division of Random House, Inc.
J. Kenji Lopez-Alt, the managing culinary director of Serious Eats, explains the bay leaf.