Pat the duck dry with paper towels.
Heat a heavy frying pan then lay the duck pieces, skin side down, in a single layer. Cook until the skins are browned and fat has been rendered.
Flip the duck pieces over and sear the opposite sides.
Lift the duck from the fat, move to a plate and set aside.
Take two tablespoons of duck fat from the frying pan and pour into a heated pot.
Saute the onion, garlic, ginger, lemongrass and kaffir lime leaves with the chili powder, turmeric and coriander until the solids are softened.
Pour in two cups of water and bring to the boil.
Drop in the seared duck and season generously with fish sauce.
Simmer for 20 minutes then drop in the sweet potato wedges and spinach.
Continue simmering for another 10 minutes then scoop out the duck and set aside.
Pour the coconut milk into the pot. Taste. Add more fish sauce, if needed. Leave to simmer, covered, while you assemble the bowls.
Divide the noodles among three bowls.
Chop the duck through the bone into manageable pieces. Divide among the three bowls.
Add sweet potato wedges to the noodles and duck in the bowls.
Ladle in the broth and spinach over the noodles, duck and sweet potatoes.
Garnish the duck curry noodle soup with sprigs of fresh Thai basil.
Serve with muoi ot and lime, calamansi or lemon on the side.