Bring a large pot of heavily salted water to a boil. Add macaroni and cook 6 minutes. Add broccoli florets and cook 2 minutes more, until pasta is al dente and broccoli is bright green and crisp-tender. Drain and set aside.

In the same pot, melt butter over medium heat. Whisk in flour and cook 1 minute. Slowly whisk in milk, then bring to a gentle simmer and cook 3 to 4 minutes, whisking often, until thick enough to coat the back of a spoon. Turn heat to low, stir in cheddar, Dijon, garlic powder, 1/4 tsp salt, and nutmeg. Stir until melted and glossy, about 1 minute. Season with black pepper to taste.

Fold the drained pasta and broccoli into the cheese sauce until evenly coated. Let sit over low heat for 1 minute, adding a splash of milk if the sauce seems too thick.

Spoon into bowls, finish with an extra grind of black pepper and more cheddar if you like, and serve hot.