Preheat oven to 400 degrees F.
Place defrosted spinach in a colander, top with several layers of paper towels, and press down firmly to get all the excess liquid out of the spinach.
Add pine nuts to a small dry skillet and toast over LOW heat until fragrant and toasted.
To a mixing bowl, add ricotta, Parmesan, garlic, salt, pepper, nutmeg, spinach and pine nuts. Stir to combine well.
Using a sharp knife, cut into the chicken breasts, slicing them across and opening them up like a book, but be careful not to cut them all the way through. Sprinkle the chicken with salt and pepper.
Spoon some of the spinach mixture into the chicken, and fold the top of the chicken breast back over the top of the stuffing.
Wrap chicken with a slice of prosciutto.
Place chicken in a cast iron skillet or a baking sheet (that's been covered with foil), and brush with olive oil.
Roast chicken for about 18-20 minutes, or until chicken reaches 165 degrees F and is cooked through.