Crumble the stock cube into a pan. Pour in 1 ltr boiling water and stir to dissolve the stock cube. Peel and thickly slice the ginger. Peel and slice the garlic. Finely slice the chilli – flick out the seeds if you want less heat.
Chuck the ginger, garlic and half the chilli into the pan. Measure out 1 tsp of the ground cinnamon and add to the pan. Cover and bring to the boil. Turn the heat down and simmer for 5 mins.
While the stock simmers, separate and finely slice the pak choi leaves. Trim and finely slice the spring onions into long, thin shreds.
Tip the beef stir-fry strips into a bowl. Drizzle in 1 tbsp oil and season. Warm a griddle or frying pan till it’s smoking hot.
Fry the beef strips for 3-5 mins till just cooked through. Transfer to a plate and loosely cover with foil. Set aside to rest.
Strain the stock through a sieve into a jug or another pan. Return the strained stock to the heat. Add 2 nests of buckwheat noodles and any thick pak choi stems. Simmer for 3-4 mins to cook the noodles.
Taste the soup and season with salt and pepper. Ladle the noodle soup into two large bowls. Sprinkle over the pak choi leaves and shredded spring onions.
Top the soup with the cooked beef strips and the remaining sliced chilli. Drizzle over the sweet chilli sauce and serve.