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Note: this is a Chinese recipe, most is on intuition.
Make sure you're meat is defrosted completely if needed, otherwise they won't become crispy. DIVIDE the meat into quite small strips, there's no need to remove any fat or muscles first.
PLACE the meat in a large bowl and cover with quite a lot of cornstarch, make sure all pieces are completely covered. It's important that the meat is as dry as possible and covered in cornstarch for a lovely crispy result.
HEAT quite a large amount of sunflower oil in a large wok on medium-high heat. Make sure you'll have enough oil to cook all the meat at one. If it doesn't fit, it's best to add less oil and cook the meat in batches instead of a too crowded pan.
If the oil is hot, but not too hot, you can add the meat. Make sure they won't stick together and continue until all the pieces are beautiful golden brown. TRANSFER the meat and oil to a sieve on top of a large bowl, you also have to remove the oil.
HEAT the wok again without the oil and add a few dashes of vinegar. Be careful: make sure you'll have a working ventilation, boiling vinegar has an unpleasant smell.
ADD about 4 to 5 tablespoons of sugar and stir until the sugar has dissolved completely. As soon as that happened, you can also add tomato ketchup. About half a bottle, more or less to taste.
The easy sweet and sour sauce is now ready. ADD the crispy pork meat back to the pan and combine well until the pork is completely coated with sauce.
HEAT about a minute before serving with some rice and a salad. Enjoy!