Scrub and peel the potatoes. Chop into chunks approximately 2cm across. Set aside. Peel and finely chop the onion. Roughly chop the tomatoes.
Warm a wok or large pan for 1 min. Add 2 tsp olive oil and the beef stir-fry strips. Fry for 3-5 mins, stirring often, till the beef has browned. Lift out and put on a plate.
Turn the heat down to medium. Add the onion with 2 tbsp water and a pinch of salt and pepper. Cook for 5 mins, stirring now and then, till the onion has softened.
While the onion fries, peel and grate the garlic and ginger. Finely grate the zest from the lemon.
Stir the garlic, ginger and lemon zest into the onion. Add the star anise and 1 tsp each mild curry powder and China 5 spice. Cook and stir for 1 min.
Return the beef to the pan with any juice from the plate. Add the potatoes and tomatoes. Pour in 200ml boiling water. Cover, turn the heat up and bring to the boil. Turn the heat down and simmer for 15 mins till the potatoes are tender.
Scoop out the star anise. Fork the spinach into the pan. Squeeze in the juice from the lemon. Taste the stew and add a pinch more salt or pepper if you think it needs it. Ladle into warm bowls and serve straight away.