Toast the cardamom pod, cloves, star anise, fennel and coriander in a small pan over medium-low heat until fragrant, 5 to 7 minutes. Place the spices onto the centre of a 20cm square piece of cheesecloth. Gather together the edges of the cheesecloth and tie with kitchen string to secure. Sear both sides of the ginger and 1/2 onion in the same pan until lightly charred, about 3 minutes on each side.
Place the turkey carcass, water, spice sachet, ginger and onion in a large pot over medium-high eat. Bring to the boil, then reduce to a simmer. Simmer for 2 hours.
Fill a large pot with lightly salted water and bring to the boil over high heat. Once the water is boiling, stir in the rice noodles; return to the boil and cook the noodles uncovered, stirring occasionally, until the noodles have cooked through, but still firm to the bite, 4 to 5 minutes. Drain well in a colander set in the sink.
Remove the carcass, spice sachet, ginger and onion from the soup. Strain the soup to remove any meat that may have fallen off the bones, if necessary. Season with fish sauce and salt. Divide the rice noodles and turkey meat evenly into 4 large bowls. Scatter the fresh basil, fresh coriander and sliced onion on top. Ladle the soup on top. Serve with a wedge of lime and hot sauce.