Chop the chiles (if you are brave enough to leave the seeds in), clean and wash the spinach and chop it too.
Heat the oil in a wok or in a suitable pan and fry the beef strips together with the chiles and garlic in small portions, stirring constantly, until all the meat is browned.
Put all the meat back into the wok, add the spinach, teriyaki sauce and sugar and fry everything for about 1 minute, continuing to stir. Now add the stock cubes, water and corn flour and heat everything while stirring until the mixture boils and thickens a little. Then season to taste and serve immediately.