Preheat oven to 400 degrees.
Chop thyme, rosemary and sage either by hand, with a small food chopper or crush them with a mortar and pestle. In a small bowl combine the herbs and softened butter.
Place turkey legs and breast and diced sweet potatoes in a roasting bag. Pour the butter and herbs over the contents in the roasting bag. Close the roasting bag and place on a baking sheet top side down.
Place in your oven and bake for 50 minutes or until the sweet potatoes are fork tender.