Step 1: Preheat Oven and Prepare Baking Dish: Preheat the oven to 375F (190C). Lightly grease a 9x13 inch baking dish with cooking spray. This helps prevent the casserole from sticking.
Step 2: Cook Pasta: Cook pasta according to package directions until al dente. Drain well and set aside. Avoid overcooking the pasta, as it will continue to cook in the oven.
Step 3: Steam or Blanch Broccoli: Steam broccoli florets until tender-crisp, about 5-7 minutes. Alternatively, blanch broccoli in boiling water for 2-3 minutes, then immediately transfer to an ice bath to stop the cooking process. Drain well.
Step 4: Prepare Cheese Sauce: In a saucepan, whisk together low-fat milk and flour until smooth. Cook over medium heat, stirring constantly, until the sauce thickens. This usually takes about 5-7 minutes.
Step 5: Season and Combine: Remove the saucepan from the heat. Stir in salt, pepper, and garlic powder. Add 3/4 cup of the shredded cheddar cheese and the Parmesan cheese, stirring until melted and smooth.
Step 6: Assemble Casserole: In a large bowl, combine the cooked pasta, diced chicken, steamed broccoli, and cheese sauce. Mix well to ensure all ingredients are evenly coated.
Step 7: Transfer to Baking Dish: Pour the mixture into the prepared baking dish and spread evenly. Sprinkle the remaining 1/4 cup of cheddar cheese and breadcrumbs (if using) over the top.
Step 8: Bake Casserole: Bake in the preheated oven for 20-25 minutes, or until the cheese is melted and bubbly and the casserole is heated through. Let stand for 5-10 minutes before serving.
Optional Tips:
- For a creamier sauce, use a combination of low-fat milk and a small amount of cream cheese.
- Add a pinch of red pepper flakes to the sauce for a hint of spice.
- Substitute other vegetables, such as cauliflower, carrots, or peas, for the broccoli.