Heat your oven to 180°C/Fan 160°/Gas 4. Slash the skin on each duck breast a few times. Measure out 1 tsp of the China 5 spice. Tip into a bowl. Stir in 1 tbsp Demerara sugar and a good pinch of salt and pepper.
Sprinkle the spice mix over the duck breasts. Rub all over, really getting into the cut bits. Flip the duck breasts over. Make sure the spices are rubbed into the bottom of the duck breasts.
Heat a frying pan or griddle pan. When it’s smoking hot, add the duck breasts, skin side down. Fry for 2 mins. Turn them over. Fry for 2 more mins.
Pop the duck breasts onto a baking tray. Put in the oven for 8 mins (10 mins for well done). Take them out. Cover with foil. Set aside for 10 mins – they’ll keep cooking as they rest.
Trim the spring onions. Finely slice the white bits. Shred the green tops. Peel and slice the ginger into matchsticks. Shred half of the Chinese leaf. Remove any tough outer leaves from the lemon grass. Trim the ends and thinly slice. Slice the lemon into wedges.
Fill the kettle and boil it. Warm 1 tsp olive oil in the pan you used for the duck. Add the white parts of the spring onions. Cook and stir for 1-2 mins to just cook the spring onions. Stir in the ginger and lemon grass.
Pour in 750ml hot water from the kettle - if your pan is too small to hold the liquid, transfer to a bigger pan or pot. Add 2 bundles of the Thai rice noodles to the pan. Simmer for 2-3 mins till the noodles are just cooked. Stir in the Chinese leaf. Season and taste.
Ladle the soup into warm bowls. Scatter the spring onion greens over the top. Slice the duck breasts. Lay them in the soup. Serve straight away with the lemon wedges for squeezing.