Place the mince, chopped watercress, Worcestershire sauce, breadcrumbs, egg yolk and the zest from one of the oranges in a large bowl. Season generously with salt and pepper. Divide into 24 pieces and roll each into a neat ball.
Heat the oil in a large frying pan, when hot, add the meatballs and saute until golden brown all over. Add the onion and saute for 2 mins to soften, then add the soy, remaining zest and all the juice of the oranges along with 300ml water. Bring to the boil, cover and simmer for 4 mins.
Mix the cornflour to a smooth paste with cold water, add to the pan and cook until meatballs are cooked through, stirring until the mixture boils and thickens. Serve the meatballs with boiled rice and broccoli.
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