Preheat the oven to 200°C/Gas Mark 6. Season the tenderloin with salt and pepper. Heat the olive oil in a large sauté pan. Sear the tenderloin until golden and transfer to an oven tray and place into the oven for 15 minutes. After 15 minutes remove the tenderloin and let it rest for at least 10 minutes.
In a saucepan add the sauce ingredients with a good splash of hot water and bring to a simmer. Cook until syrupy and reserve.
In the same pan in which you pan-fried the tenderloin add the peppers, shallot and pak choi and leek and sauté until the leeks are soft. At that point pour the hot and sour sauce onto the vegetables.
Serve on a large plate and carve the tenderloin to place on top of the hot and sour vegetables.
Serve with steamed rice and a wedge of lemon (optional).