Arrange chicken in a single layer in the prepared casserole dish.
In a medium bowl, whisk together heavy cream, Parmesan cheese, chopped spinach, minced garlic, nutmeg, Italian seasoning, and remaining salt and pepper. Stir until smooth and evenly combined.
Pour the creamy spinach mixture evenly over the chicken, making sure each piece is coated and sauce settles around the edges.
Cover loosely with foil and bake for 20 minutes.
Remove foil and continue baking uncovered for 10–20 more minutes, until chicken reaches 165°F and the sauce is bubbling gently and slightly thickened.
Let rest 5–10 minutes before serving. Spoon sauce over each piece when plating. Garnish with parsley if desired.
Serve with mashed potatoes, buttered rice, or simple green beans.
