Reduce oven temperature to 350° F.
Bring a large pot of salted water to a boil. Add the pasta and cook according to package directions for al dente, adding the broccoli florets to the pot during the last 1-2 minutes of the cook time. Drain the pasta and broccoli into a colander, rinse under cold water, drain. Put the cooked pasta and broccoli back into the empty cooking pot. Set aside.
Heat the butter and olive oil in a large skillet over medium heat. Add the onion and cook until softened, about 3 minutes.
Add the garlic and flour and cook for 30 seconds, stirring.
Stir in the chicken broth, milk, garlic powder, onion powder, dried dill, salt and pepper. Cook, stirring often, until the mixture just comes to a simmer and thickens slightly. Remove pan from the heat.
To the pot with the pasta and broccoli, add the shredded cooked chicken, half of the shredded cheese, ⅔ of the chopped cooked bacon and the sauce from the skillet. Stir to combine and then transfer the mixture to a 13x9-inch baking dish.
Top casserole with the remaining shredded cheese and remaining chopped bacon. Cover with foil (spray the underside of the foil with cooking spray to help prevent the cheese from sticking to it). Bake, covered, for 20 minutes. Then uncover and bake 5-10 minutes more, until cheese is melted and casserole is hot throughout.
Serve with chopped fresh chives.