The original recipe came from the BBC's Chinese Food Made Easy, and used beef fillet, but I couldn't find any decent fillet steak that day, so I used a beautiful pork fillet instead, which worked beautifully.
Press each round of pork with your fingers, a meat mallet, or the flat of a knife until half the thickness, then slice into thin pieces. Place in a bowl and toss with 1/2tbsp light soy, the oyster sauce, sugar, salt and pepper, then place to one side.
Heat your wok until smoking before adding 1 tbsp groundnut oil, then the garlic and chopped chilli. Stir fry for a few seconds, then add the spinach and toss until warmed through. Pile onto a warm serving plate.
Bring the wok back to temperature and add another 1 tbsp groundnut oil, then stir-fry the pork for 2-3 minutes, or until cooked as you like.
Pour in the sherry and sprinkle over the chilli flakes. Pile onto the spinach leaves.
Bring the wok back to temperature and add the remaining 1tbsp groundnut oil. Add the mushrooms and stir-fry for one minute.
Add a splash of water to steam the mushrooms for another minute or so, then season to taste with balsamic and light soy sauce. Spoon onto pork and serve.