For the stir-fry sauce, mix the ingredients in a small bowl. Heat the vegetable oil in a wok or large frying pan over high heat. Add the beef and spread it out in the pan. Leave it to sear for 5-6 minutes or until very darkly coloured. Then mix and break up the beef with your spatula. Now add the garlic and onion and stir-fry for another 2 minutes. Add the tomatoes and simmer for 15 minutes. In the meantime, cook the noodles until just al dente. If the sauce is not ready, rinse them under cold running water to stop them cooking. If the sauce is ready, transfer them straight into the sauce. Toss the cooked noodles through the sauce. Divide among serving bowls and sprinkle with spring onions and chilli flakes, if using.