Preheat oven to 350°F. Grease a 13x9-inch dish; set aside.
Cook pasta in a large pot of salted boiling water for 1 minute less than the package instructions for al dente. Drain; set aside.
Meanwhile, cook the bacon in a Dutch oven or very large skillet over medium heat until crispy, about 7 minutes. Remove the bacon to a paper towel-lined plate, reserving the drippings in the pot.
Add the onion and celery to the drippings in the pot. Sauté the vegetables over medium-high heat until soft, about 5 minutes. Stir in the garlic and cook for 30 more seconds. Remove from the heat. Add the cooked, drained pasta, the cooked chicken, and the thawed broccoli to the pot.
In a small bowl, whisk together the Alfredo sauce, parsley, dill, chives and onion powder. Pour the Alfredo sauce over the pasta mixture and toss to coat. Taste and season with salt and pepper, if desired.
Transfer the pasta mixture to the prepared dish. Sprinkle the cooked bacon on top and finish with a layer of shredded cheese.
Bake for about 20-25 minutes, or until the filling is hot and bubbly and the cheese is melted.
Garnish with fresh herbs and serve!