Peel the ginger and the garlic, then slice thinly with a sharp knife. Cut the chilli in half. Finely grate the lemon zest.
Pour 1 ltr boiling water into a large pan. Bring back up to the boil. Crumble in the stock cube and stir to dissolve.
Drop the sliced ginger and garlic into the pan, along with 1 chilli half, all the lemon zest, 2 star anise and 1 cinnamon stick. Cover. Simmer for 20 mins, stirring occasionally.
While the stock simmers, thinly slice the shiitake mushrooms. Use a vegetable peeler to peel the carrots into thin ribbons. Add them both to the stock, along with 2 bundles of noodles, and simmer for 3 mins.
While the veg and noodles simmer, shred the pak choi. Stir it into the broth. Cook for a further 1 min.
Finish the pho by squeezing in the juice from half the lemon and stirring in 1 tbsp tamari.
Ladle the pho into deep bowls. Slice the remaining chilli half and the spring onions and scatter over the top. Serve with lemon wedges and the remaining tamari on the side.