Grease a 9x13" casserole dish or baking pan with non-stick cooking spray.
Season both sides of chicken with salt and pepper, then lightly dust each side with flour.
In a large skillet over medium heat, melt 2 tablespoons of butter.
Add the chicken and cook until golden brown, about 3-5 minutes per side (3 for cutlets, 5 for entire breast).
Place cooked chicken on a plate and cover with foil to keep it warm.
Melt remaining 2 tablespoons of butter in the same skillet over medium heat. Add the onions and sauté for 4 minutes. Add garlic and sauté another 1 minute.
Add wine (or stock) to the skillet and stir to pick up any browned bits from bottom of pan. Bring to a boil and reduce wine by half. Add the cream and bring to a boil then reduce by half.
Add the fresh spinach and any accumulated juices from chicken; continue cooking until spinach wilts. Simmer until sauce thickens again (See Note 2). Season to taste. Preheat oven broiler.
Arrange sautéed chicken in bottom of pan. Spoon spinach sauce mixture on top of chicken and evenly distribute mozzarella cheese on top. Place under the broiler for several minutes until cheese is bubbly and golden brown. Garnish with nutmeg and parsley before serving.
If breasts are large, slice horizontally in half to make chicken cutlets.
Feel free to substitute two 10 ounce packages of thawed and drained frozen cut-leaf spinach for the 1 pound fresh spinach. I prefer fresh spinach, but it takes longer to reduce sauce. Using frozen is easier and quicker.