Cut a deep slit along one side of each chicken breast, without slicing right through, creating a pocket and then open it like a book. Finely chop 4 sage leaves and then mix into the ricotta and season with salt and pepper.
Lay two Prosciutto ham slices on a clean chopping board, overlapping them slightly. Put a sage leaf in the middle and lay an open chicken breast on top. Spoon a quarter of the ricotta mixture onto the middle of the chicken, then fold the sides together again, to enclose the filling. Now wrap the Prosciutto ham slices around the stuffed chicken breast. Wrap in cling film. Repeat with the rest of the chicken breasts and chill for 1-2 hours to firm up slightly.
Pre-heat the oven to 180 degrees centigrade and place a roasting pan.
Take a heavy-based frying pan and add the olive oil. When hot, pan sear the Prosciutto-wrapped chicken for 2 minutes on each side, until brown. Lay a few thyme sprigs on each chicken breast and place in the hot roasting pan in the oven. Cook in the oven for 8-10 minutes, depending on size, or until the meat feels just firm when lightly pressed.
Rest the chicken, covered with foil for 2-3 minutes. Slice each stuffed chicken breast diagonally to get thick slices and arrange on a plate. Serve with a full-bodied red wine glaze, steamed greens and light mashed potatoes or a zesty couscous.