Cut the tomato in two, scoop out the seeds and juice and keep in a bowl. Finely chop the actual tomato and keep aside in another bowl.
Add the soy sauce, oyster sauce and chilli flakes to the tomato juice and mix.
Finely dice the onion and cook in a non-stick frypan. After a minute, roughly chop up the beetroot stalks and add to the pan. While the onion and stalks are cooking, chop up the feta and herbs and add to the chopped tomato.
Chop the beetroot leaves and add to the onion for the last minute of cooking, or until the leaves start to wilt. Combine the onion mixture with the tomato and feta.
Crack four eggs into a bowl and add the water. Whisk until combined. Heat a little oil in your frypan over high heat, then add the eggs. Tilt the pan around so the mixture completely covers it. Cook for a few minutes until the mixture is golden and starting to set. Test the edges of the omelette, then pour the filling over one half and fold over the other half of the omelette to enclose the filling.
Serve immediately on two plates and pour over the sauce.