Bring a large pot of water to boil. Meanwhile, chop the broccoli crowns into florets. Peel the tough outer skins of the broccoli stems using a vegetable peeler, then chop the stems into ½-inch discs. Add the broccoli to the boiling water and cook for 3-4 minutes, or until fork-tender. Alternately, you can steam the broccoli in a steam basket over the boiling water. Use a skimmer or strainer to remove the broccoli from the pot, leaving the water.

Maintain the water at a boil. Add the pasta to the pot and cook according to the package directions until al dente. Reserve 1½ cups of the pasta water and then drain the pasta well. Set aside.

Return the pot to the stove over medium-high heat. Add the butter and olive oil and heat until the butter is melted. Add the onion and cook for about 5 minutes, stirring occasionally, until the onion is tender. Stir in the garlic, red pepper flakes and salt and continue cooking another 1-2 minutes. Return the broccoli to the pot and cook for 1-2 additional minutes. Taste and season with pepper as desired.

Transfer the broccoli mixture to a food processor or blender. Pulse, scraping down as needed, until the mixture is finely chopped. Add the cream or yogurt and process until smooth, adding reserved pasta water a little at a time to reach your desired consistency. Season with freshly ground black pepper to taste.

Return the pasta to the pot and toss with the sauce. Serve with freshly grated Parmesan.