Measure out 1 tbsp of the tamari and pop it in a dish with 1 tbsp brown rice vinegar. Add the beef stir-fry strips and toss to mix, then sprinkle in 1 tbsp cornflour. Toss to coat the beef. Set aside while you prepare the veg.
Peel and finely slice the onion. Halve the pepper and scoop out the seeds and white pith. Finely slice it. Trim the roots and the top 1cm off off the salad onion. Finely slice it, keeping the white and green parts separate. Slice the spring green leaves off their thick central cores, then shred the leaves into fine ribbons.
Finely slice the chilli, flicking out the seeds and white pith for less heat. Peel and grate the garlic and ginger.
Tip the rice into a sieve and give it a good rinse. Tip it into a small pan and add 300ml boiling water and a pinch of salt. Cover, bring to the boil, then turn the heat right down. Gently simmer for 8 mins, till all the water has been absorbed. Take off the heat and leave to steam in the pan, with the lid on, for 5-10 mins.
While the rice simmers, put a wok or deep frying pan on a high heat for 2 mins. Add ½ tbsp oil and the beef. Stir fry for 4-5 mins till the beef is browned all over. Lift out of the wok and pop onto a plate.
Add another ½ tbsp oil to the wok. Add the onion, red pepper, the white parts of the salad onion and the spring green leaves. Stir fry for 5 mins till the veg have started to soften.
Add the beef back to the wok. Stir in the chilli, garlic and ginger. Add the remaining tamari, 1 tbsp brown rice vinegar and the tamarind paste. Pour in 300ml boiling water. Simmer for 8-10 mins till the veg and beef are tender.
Fluff the rice up with a fork and divide between 2 warm bowls. Top with the Singapore beef stir-fry garnish with the green parts of the salad onion and serve straight away.