Preheat and prep: Heat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with nonstick spray or butter.

Slice the zucchini: Cut zucchini into 1/4-inch half-moons. Toss with a pinch of salt and place in a colander to drain for 10–15 minutes. Pat dry with paper towels. This prevents a watery casserole.

Sauté aromatics: In a large skillet, warm olive oil or butter over medium heat. Add onion and cook until soft, 4–5 minutes. Stir in garlic for 30 seconds, just until fragrant.

Build the Alfredo base: Pour in chicken broth and heavy cream. Add cream cheese in small pieces, stirring until smooth. Mix in Parmesan, Italian seasoning, and red pepper flakes if using. Simmer gently 2–3 minutes until slightly thickened. Season with salt and pepper.

Par-cook zucchini: Add the zucchini to the skillet and toss in the sauce for 2–3 minutes. You want it coated but not fully soft. Stir in the cooked chicken. If using lemon zest, fold it in now for a bright note.

Assemble the casserole: Transfer the mixture to the prepared baking dish. Top evenly with mozzarella and a sprinkle of extra Parmesan. If you like crunch, mix bread crumbs with a teaspoon of olive oil and scatter on top.

Bake: Bake for 20–25 minutes, until bubbling and lightly golden. For a deeper crust, broil for 1–2 minutes at the end, watching closely.

Rest and serve: Let it rest 8–10 minutes before serving. This helps the sauce set. Garnish with chopped parsley.