Preheat oven broiler and lightly grease a 9×13-inch baking dish.

Season chicken breasts with salt and pepper. Lightly dredge both sides in flour.

Heat 2 tablespoons of butter in a skillet over medium heat. Sear chicken 3–5 minutes per side (3 minutes for cutlets, 5 for whole breasts) until golden brown. Transfer to a plate and cover with foil.

In the same skillet, melt the remaining 2 tablespoons of butter. Sauté diced onion for 4 minutes, then add garlic and cook for 1 minute more.

Pour in chicken broth, scraping up browned bits. Bring to a boil and reduce by half. Stir in the heavy cream, bring to another boil, and reduce again by half.

Add fresh spinach and juices from the cooked chicken. Stir until spinach wilts. Simmer sauce until thickened to desired consistency. Adjust seasoning if needed.

Arrange cooked chicken in the prepared baking dish. Pour spinach cream sauce over chicken and sprinkle with shredded mozzarella.

Broil for a few minutes until cheese is melted and golden brown. Garnish with nutmeg and parsley. Serve immediately.