Cut a deep slit along one side of each chicken breast, without slicing right through, then open it out like a book. On a clean cutting board, mince 4 sage leaves, then mix into the ricotta and season with salt and pepper to taste.
Lay two prosciutto slices on the 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 slices around the stuffed chicken breast. Wrap in plastic wrap. Repeat with the rest of the chicken breasts and chill for 1 to 2 hours to firm up slightly.
Heat the oven to 180°C and place a roasting pan in the oven to heat up. Heat a heavy skillet and add the olive oil. When hot, pan-fry the prosciutto-wrapped chicken, in batches if necessary, for 2minutes on each side until browned. Lay a few thyme sprigs on each chicken breast, then place in the hot roasting pan. Cook in the oven for 12 to 15minutes, depending on size, or until the meat feels just firm when lightly pressed.
Rest the chicken, covered with foil, in a warm place for 5 to 10minutes. Slice each stuffed breast thickly on the diagonal and arrange on warm plates. Serve with steamed greens and light mashed potatoes or a zesty couscous.