To make the marinade and sauce: In a large bowl, combine the garlic, yellow bean sauce, pork fillet and Shaohsing rice wine with the Sweet and Sour Sauce ingredients except for the groundnut oil. Marinate the pork for as long as possible.
Before cooking, strain and slice the pork fillet. Retain the marinade.
Heat a wok over high heat and add the 2 tablespoons groundnut oil. Add the sliced pork and stir-fry until browned for about 3 to 4 minutes. Lastly, pour in the marinade and reduce the heat. Cook so the sauce achieves a sticky consistency, a further 2 to 3 minutes.
To make the onions: Heat a small wok or pan, add the groundnut oil and stir-fry the red onions until translucent and softened, about 4 to 5 minutes. Then, add the black rice vinegar, and brown sugar, and caramelize for another minute.
For service: Heat a griddle pan, add some groundnut oil, and place the sliced seeded buns, cut side down, and grill until browned slightly. Stuff the buns with some of the salad leaves, some of the sticky pork slices, caramelized onions, and sliced red tomatoes. Serve immediately.