Preheat oven to 400°F.
Toss cauliflower with olive oil, salt, and pepper. Roast 12–15 minutes until lightly golden.
Reduce oven heat to 375°F.
Add chicken to a large bowl and season with salt and pepper.
In a saucepan, heat heavy cream, cream cheese, broth, lemon zest, lemon juice, garlic powder, and rosemary until smooth. Add Parmesan if using.
Combine roasted cauliflower, chicken, and the lemon rosemary sauce.
Transfer to a greased baking dish and top with mozzarella.
Bake 20–25 minutes until bubbly and chicken is fully cooked.
Optional: garnish with lemon slices and rosemary