Instructions

1. Bring a large pot of heavily salted water to a boil over medium-high heat. Meanwhile, cut 1 1/2 pounds broccoli crowns or florets into 1-inch pieces, and cut any large pieces of stalk crosswise into 1/2-inch-thick pieces (8 to 9 cups total). Grate 6 ounces sharp cheddar cheese on the large holes of a box grater (or measure out 1 1/2 cups store-bought grated). Grate 2 tablespoons Parmesan cheese on the smallest holes of the grater if needed.
2. Add the broccoli to the boiling water and cook until bright green and crisp-tender, about 3 minutes. Drain.
3. Wipe out the pot, add 3 tablespoons unsalted butter, and return to medium heat. When the butter is melted, add 2 tablespoons all-purpose flour and 1/2 teaspoon ground mustard and whisk for 1 minute to cook the floury taste out. While whisking constantly, add 1 cup whole or 2% milk. Bring to a full simmer, whisking often.
4. Add the cheddar, Parmesan, and 3/4 teaspoon kosher salt. Whisk until the cheeses are completely melted, the sauce will be thick. Add the broccoli and gently fold until coated in cheese sauce. Taste and season with more kosher salt as needed.