Wring out defrosted spinach in clean kitchen towel.
Toast nuts and combine with spinach in a bowl, mix in cheeses, garlic, salt, pepper and nutmeg.
Cut into and across but not all the way through the chicken breasts, opening them up like a book with a sharp knife. Season the chicken with salt and pepper. Fill each with a small mound of stuffing. Flap chicken breasts back over stuffing and wrap each breast with prosciutto to seal them, carefully covering the whole breast. Brush chicken all over with some extra-virgin olive oil and roast 18 to 20 minutes until cooked through.