Tip the rice into a bowl, cover with cold water and set aside to soak.
Crumble half the stock cube into a heatproof jug. Stir in 200ml boiling water to dissolve the stock cube. Add 1 tbsp of the tamari (keep the rest for later), 2 tbsp brown rice vinegar and 1 tbsp of the honey. Stir to mix. Set aside.
Put a wok or deep frying pan on a medium-high heat and warm it up for 2 mins. Add ½ tbsp oil and the meatballs. Fry for 6-8 mins, turning once or twice, till the meatballs are browned all over.
While the meatballs fry, peel and finely slice the onion. Trim and peel the carrot and slice it on the diagonal. Halve the pepper, scoop out the seeds and white pith, and slice it into batons.
Drain the rice and tip it into a small pan. Add 300ml boiling water and a pinch of salt. Cover, bring to the boil, then turn the heat down and simmer for 8 mins till all the water is absorbed. Take off the heat and leave the rice in the pan for 3-5 mins to steam and finish cooking.
Scoop the meatballs out of the pan and pop them on a plate. Drain off any excess liquid. Add the onion, carrot and pepper to the wok with 2 tbsp water. Steam fry for 5 mins till the veg start to soften.
Tip the meatballs back into the pan and pour in the flavoured beef stock. Pop on a lid (if you don’t have a lid, use a baking tray to cover the pan) and simmer for 8 mins to cook the meatballs through.
While the meatballs simmer, trim the dry ends off the cavolo nero and finely shred it. Fork it into the wok with the meatballs and stir fry for 3-4 mins till the cavolo nero is just wilted and tender. Stir in 1 tbsp sesame oil.
Fluff the rice and scoop it into 2 warm bowls. Top with the sweet and sour meatballs and veg and serve straight away.