Real Sweet-and-Sour Pork (Gulao Rou)
In a small bowl, combine 1 cup cornstarch and 1/2 cup water to make a smooth paste. Place 1/2 cup cornstarch in a separate bowl. Dip the cubes of pork into the wet mixture, then roll them in the dry cornstarch.
Place a wok over high heat and add the oil. Test the oil by dropping a pork cube in the oil; it should sizzle immediately. Add all the pork cubes and deep-fry until golden. Remove them with a slotted spoon and drain them on paper towels. Reduce the heat under the wok while the pork cools for 3 to 4 minutes. Then return the heat to high, add the pork cubes, and fry again for 1 minute. Remove them from the oil and drain again on paper towels.
In a small bowl, mix together the ketchup, vinegar, sugar, and salt. In another bowl, dissolve the teaspoon of cornstarch in 1/4 cup water. Set both bowls within easy reach of the stove.
Place 2 tablespoons of the oil used for deep-frying (strain it first if necessary) in a clean wok over medium-high heat. Add the leek and garlic and stir-fry until fragrant, then add the ketchup mixture and simmer for 1 minute. Add the cornstarch solution and cook for another minute, stirring until smooth and thick. Add the pork, pineapple, and green pepper and stir to coat thoroughly with the sauce. Remove from the heat and serve promptly.