Step 1

Heat the oven to 375°F. Trim 1 pound broccoli. Keeping the stems and florets separate, dice the stems, then cut the florets into 1-inch pieces.

Step 2

Heat 1 tablespoon of the olive oil in a Dutch oven or broiler-safe large pot over medium heat until shimmering. Add the broccoli florets and 1/4 teaspoon of the kosher salt. Cook until the broccoli brightens in color, about 3 minutes. Transfer to a plate.

Step 3

Add 1 tablespoon of the olive oil to the pot. When shimmering, add the broccoli stems and 1/4 teaspoon of the kosher salt. Cook until softened and starting to brown, 2 to 3 minutes.

Step 4

Add the remaining 1 tablespoon olive oil, 1 diced small yellow onion, and 1/4 teaspoon of the kosher salt. Cook, stirring often, until softened but not browned, about 5 minutes. Stir in 4 thinly sliced garlic cloves, 1/2 teaspoon ground mustard, and 1/2 teaspoon red pepper flakes if using. Cook until fragrant, about 1 minute.

Step 5

Stir in 1 pound dried short pasta, 2 1/4 cups water, 2 cups whole milk, 1 (12-ounce) can evaporated milk, and the remaining 1 1/4 teaspoons kosher salt. Cover and transfer the pot to the oven. Bake, stirring halfway through to break up any clumps of pasta, until the pasta is almost tender, 18 to 20 minutes total.

Step 6

Remove the pot from the oven. Heat the broiler on high. Stir 6 ounces of the shredded sharp cheddar cheese (1 1/2 cups) and the reserved broccoli florets into the pasta until the cheese is melted. Taste and season with kosher salt and black pepper as needed.

Step 7

Evenly sprinkle with the remaining 2 ounces shredded cheddar cheese (1/2 cup). Return to the oven and broil uncovered until the cheese on top is melted and brown in spots, 3 to 5 minutes.