In a large pot bring water to a boil and add the pasta. Cook according to package directions. Add the broccoli to the boiling water the last 3 minutes of cooking. Drain and set aside.
In the same pot, add the butter and melt over medium heat. Whisk in the flour and onion powder and cook for about a minute to create a roux. Slowly whisk in the milk and cheese and bring to a boil over medium high heat until it starts to thicken.
Add in pasta shells, broccoli, and chicken and toss to coat in the sauce. Serve immediately.