Preheat and prep: Set your oven to 400°F (200°C). Lightly grease a 9x13-inch baking dish.
Season the chicken: Pat chicken dry and season with 1 teaspoon salt, 1/2 teaspoon black pepper, and half the Italian seasoning.
Toss to coat.
Sauté aromatics: Warm 1 tablespoon olive oil in a large skillet over medium heat. Add onion and cook 3–4 minutes until softened. Stir in garlic and cook 30 seconds until fragrant.
Brown the chicken: Add remaining 1 tablespoon olive oil to the skillet.
Add chicken and cook 4–6 minutes, stirring, until lightly browned on the outside. It doesn’t need to be fully cooked through. Transfer chicken and onions to the baking dish.
Prep the zucchini: Add sliced zucchini to the same skillet with a pinch of salt.
Sauté 3–4 minutes just to soften slightly and drive off moisture. Spread over the chicken in the baking dish.
Make the sauce: Lower heat and whisk in cream cheese, heavy cream, and chicken broth. Stir until smooth.
Mix in Parmesan, remaining Italian seasoning, and red pepper flakes if using. Taste and adjust salt and pepper.
Assemble: Pour the creamy sauce evenly over the chicken and zucchini. Sprinkle mozzarella on top.
Bake: Place in the oven for 18–22 minutes, until the cheese is melted and bubbling and the chicken reaches 165°F (74°C).
Broil for color (optional): Broil 1–2 minutes to lightly brown the top.
Watch closely.
Rest and garnish: Let the bake rest 5 minutes to set. Finish with chopped parsley or basil. Serve warm.