Preheat oven to 400°F. Lightly grease a 9x13-inch glass casserole dish with olive oil or butter.
Pat chicken breasts dry. Season both sides with ½ teaspoon salt and ¼ teaspoon black pepper.
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.
