Preheat and prep: Heat your oven to 375°F (190°C).
Grease a 9x13-inch baking dish.
Par-cook the vegetables: In a large skillet, warm olive oil or butter over medium heat. Add onion and cook 2–3 minutes until softened. Stir in broccoli, cauliflower, zucchini, and bell pepper with a pinch of salt and pepper.
Cook 5–6 minutes until just crisp-tender. Add minced garlic in the last minute. Transfer to a large bowl.
Make the creamy base: In the same skillet over low heat, add heavy cream and cream cheese.
Whisk until smooth and slightly thickened, 3–4 minutes. Stir in Italian seasoning, garlic powder, smoked paprika, and half of the Parmesan. Taste and adjust salt and pepper.
Combine with chicken: Add the cooked chicken and the sautéed vegetables to the bowl.
Pour the creamy sauce over everything and toss to coat evenly.
Assemble the casserole: Spread the mixture into the baking dish. Top with shredded cheese and the remaining Parmesan.
Bake: Place in the oven for 18–22 minutes, until the edges bubble and the top is melted and lightly golden.
Finish and serve: Let it rest 5–10 minutes so it sets. Sprinkle with chopped parsley and serve warm.