Marinate the beef slices with mirin and light soya sauce. Set aside.
In a frypan, heat 1 tbsp (3 tsp) of oil. Add garlic, red bell pepper and carrots and stir fry for a while. Then add onions and stir fry till translucent. Lastly, add mushrooms, spinach and spring onions and stir fry until spinach is wilted. Remove from pan.
In a clean frypan, heat 2 tsp of oil. Add marinated beef slices and stir fry till cooked. Remove from pan.
In a saucepot, bring water to a boil and add 250g of sweet potato noodles. Cook for about 6-7 minutes until noodles are soft but slightly chewy. Remove from pot.
Add the noodles, stir-fried vegetables and meat into a big bowl. Add the sauce and mix everything together. Add more soya sauce and sugar if needed.