Preheat the oven to 400 degrees F (200 degrees C).
Heat a roasting pan on the stove and sear the duck skin side down until some oil comes out of it.
Turn the duck legs over so they are skin side up and remove the pan from the heat.
Chop the potatoes into slices or small cubes if you are using them, and arrange them around the duck legs in the roasting dish.
Add some sprigs of thyme, salt and black pepper to taste.
Roast for 1 1/2 to 2 hours, turning the potatoes (if using) occasionally.
When the duck is done and the potatoes are crispy and tender, serve hot.