Coat the marinated pork shoulder with cornstarch and sweet potato starch in order, then let it sit to absorb moisture.

Heat oil and quickly fry the pork slices until golden brown on the outside, then set aside. (A simple way to check the oil temperature: dip a chopstick in a little sweet potato starch and put it in the oil. If the starch moves quickly, the oil is hot enough.)

In another pan, add a little oil, minced garlic, and salt. Stir-fry the bell peppers, then set aside.

In the same pan from step 3, add the prepared sweet and sour sauce and bring to a boil. Then add the bell peppers, fried pork, and pineapple. Stir-fry until everything is coated and colored, then serve.