Preheat and prepare the dish. Heat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with nonstick spray or butter.
Cook the pasta. Boil in generously salted water until just shy of al dente, about 1–2 minutes less than package directions. Drain and set aside.
Keep it slightly underdone so it finishes perfectly in the oven.
Sauté the aromatics. In a large skillet, warm butter and olive oil over medium heat. Add garlic and cook until fragrant, about 30–45 seconds. Don’t let it brown.
Make the roux. Sprinkle in the flour and whisk for 1 minute to remove raw flour taste.
Build the sauce. Slowly add chicken broth while whisking, then stir in the cream.
Simmer gently for 3–4 minutes until slightly thickened.
Season and cheese it. Stir in Parmesan, nutmeg, and red pepper flakes if using. Season with salt and pepper. The sauce should be silky and flavorful.
Add the good stuff. Fold in chicken, spinach, and roasted red peppers.
The spinach will wilt slightly in the hot sauce.
Combine with pasta. Add the cooked pasta and half the mozzarella. Toss to coat everything evenly.
Assemble. Transfer to the prepared baking dish. Top with remaining mozzarella, a light shower of Parmesan, and panko breadcrumbs.
Finish with parsley.
Bake. Place on the middle rack and bake 18–22 minutes, until bubbly and the top is golden. For extra color, broil for 1–2 minutes at the end—watch closely.
Rest and serve. Let it sit for 5–7 minutes so it sets slightly. Scoop into bowls and enjoy while hot.