Bring water to a boil. Add the macaroni and cook according to the package directions, adding the broccoli during the last 2 minutes of cooking and drain.
Melt the butter in a large saucepan over medium heat. Stir in flour, onion powder, salt and pepper. Add the milk and stir until slightly thickened, about 5 minutes. Add cheddar cheese, continue stirring, over low medium heat until completely melted, about 2 minutes.
Add drained macaroni and broccoli into the cheese sauce, stir for about 2 more minutes. Remove from heat. Transfer to a serving bowl.