Bring a large pot of salted water to a boil. Add the pasta and cook it just until al dente, according to package instructions. Add the broccoli to the pot during the last 3 minutes of the pasta’s cooking time. The broccoli is done when it's bright green and crisp-tender. Reserve 1 cup of the pasta water. Drain the pasta and broccoli and set aside.

Return the pot to the stovetop and melt the butter over medium heat. Turn the heat to low and whisk in the flour to create a paste. Whisk in milk until smooth. Turn the heat to medium and whisk until the sauce starts to thicken, about 3 to 5 minutes.

Stir in the cheddar cheese until it is melted into the sauce. Add the ground mustard, garlic powder (if using), salt, and pepper.

Add the drained pasta and broccoli and stir until well coated with the cheese sauce. If the sauce is too thick, add a little of the reserved pasta water and stir to combine.

Ladle into bowls and serve with freshly grated Parmesan cheese and crushed red pepper flakes, if desired.