Step 1: Preheat and Prepare
Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish with cooking spray or butter and set aside.

Step 2: Cook the Pasta
Bring a large pot of salted water to a boil. Add the pasta and cook for 2 minutes LESS than the package directions (it will finish cooking in the oven). Drain and set aside. Do not rinse.

Step 3: Prepare the Broccoli
If using fresh broccoli, add the florets to boiling water for 2-3 minutes until bright green and slightly tender. Drain immediately. If using frozen, thaw completely and squeeze out excess moisture with paper towels or a clean kitchen towel.

Step 4: Make the Alfredo Sauce
In a large saucepan over medium heat, melt the butter. Add minced garlic and sauté for 30-60 seconds until fragrant but not browned. Pour in the heavy cream and chicken broth, stirring to combine. Bring to a gentle simmer.

Step 5: Add the Cheese
Reduce heat to medium-low. Gradually add 1 cup of freshly grated Parmesan cheese, whisking constantly until melted and smooth (about 2-3 minutes). Stir in Italian seasoning, salt, pepper, and red pepper flakes if using. Taste and adjust seasoning.

Step 6: Combine Everything
In a large mixing bowl, combine the cooked pasta, chicken, broccoli, ½ cup Parmesan cheese, and 1 cup of the shredded mozzarella. Pour the Alfredo sauce over everything and stir gently but thoroughly until everything is evenly coated.

Step 7: Transfer to Baking Dish
Pour the mixture into your prepared 9×13-inch baking dish, spreading it out evenly. The dish will be quite full, which is perfect.

Step 8: Add the Cheese Topping
Sprinkle the remaining 1 cup of mozzarella cheese and ¼ cup Parmesan cheese evenly over the top. The generous cheese layer will create that golden, bubbly crust everyone loves.

Step 9: Bake Covered
Cover the baking dish tightly with aluminum foil (spray the underside with cooking spray to prevent cheese from sticking). Bake for 20 minutes.

Step 10: Bake Uncovered
Remove the foil and continue baking for an additional 10-15 minutes until the cheese on top is golden brown and bubbly, and the edges are lightly crispy.

Step 11: Rest and Serve
Remove from the oven and let the casserole rest for 5 minutes before serving. This allows the sauce to thicken slightly and makes serving easier. Garnish with freshly chopped parsley.

Step 12: Enjoy!
Serve hot and enjoy the cheesy, creamy goodness! Store any leftovers in the refrigerator for up to 4 days.