Prep the oven and pan: Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish with olive oil or butter.

Slice and sweat the zucchini: Cut the zucchini into thin rounds (about 1/8–1/4 inch). Lay them out on paper towels, sprinkle lightly with salt, and let them sit 10–15 minutes. This draws out moisture.

Pat dry: Blot the zucchini well with fresh paper towels. Removing excess water helps prevent a watery casserole.

Sauté aromatics: Warm 1 tablespoon olive oil or butter in a skillet over medium heat. Add onion and cook 3–4 minutes until soft. Stir in garlic and cook 30 seconds until fragrant.

Make the creamy sauce: Lower the heat. Add cream cheese to the skillet, breaking it up with a spatula. Stir in heavy cream and chicken broth, whisking until smooth. Add Italian seasoning, red pepper flakes (if using), 1/4 teaspoon salt, and a few grinds of pepper. Simmer 2–3 minutes to thicken slightly. Stir in parmesan.

Toss the chicken: Add the shredded chicken to the sauce. Stir to coat evenly and warm through. Taste and adjust seasoning.

Layer the casserole: Spread half the zucchini slices in the prepared baking dish. Spoon half the creamy chicken mixture over the zucchini. Sprinkle with half the mozzarella. Repeat with remaining zucchini, chicken, and mozzarella.

Bake: Cover the dish loosely with foil and bake 15 minutes. Remove the foil and bake another 10–15 minutes, until bubbly and lightly golden on top.

Rest and garnish: Let the casserole rest 8–10 minutes before serving. Top with chopped parsley or basil for color and freshness.

Serve: Scoop generous portions and enjoy as-is, or pair with a crisp side salad.