Place the turkey between 2 sheets of cling film and flatten with a heavy pot or meat mallet.
Wash the sage and pull 6 leaves from the stalks. Slice the ham diagonally. Put a piece of ham and a sage leaf on each turkey portion.
Fold the turkey and secure with cocktail sticks, season with salt and pepper.
Peel and finely dice the onion. Wash the spinach and drain in a sieve.
Heat a grill pan and fry the stuffed turkey for 3-4 minutes on each side.
Meanwhile, heat the olive oil in a pan and saute the onion until softened. Add the spinach and stir until wilted. Season with salt and pepper.
Season the turkey with Balsamic vinegar and serve with the spinach.