Heat the oil and stir-fry the spring onions until
softened. Add the beef and stir-fry until just
browned.
Stir in the stock, bring to the boil, cover
and simmer for 2 minutes. Mix the corn-flour and
water, stir into the pan and simmer, stirring, until
the sauce thickens.
Stir in the tomatoes and simmer
just until they are heated through.