Combine all marinade ingredients and marinate the pork for at least 1 hour.
In a medium pan, add your the vegetable oil and heat to medium-high heat (around 180 to 200C)
Drain excess juices from the pork marinade. Coat each pork slice with cornstarch until a dry, thin layer forms around it.
Once your oil is hot, fry the pork slices until cooked through. It should be somewhat golden and takes about 3 minutes per side. If needed, fry pork in batches so you don't overcrowd the pan (it won't get crispy). Set aside on a paper towel or wire rack so they don't get soggy. Repeat until all your pork are fried.
Remove oil from the pan and set aside if preferred. Wipe out the excess oil and starch from the pan (or you can wash it too)
In a bowl, combine all sweet and sour ingredients and mix until well-combined. Adjust taste to preference.
In the pan, heat to medium and cook the onions until semi-translucent, about 3 minutes.
Add in your chopped bell peppers and stir fry until cooked through, about 3 to 5 minutes.
Adjust heat to low and add in your combined sweet and sour sauce to the pan and until sugar has been dissolved completely. Let simmer for about 2 minutes. Add water if it's starting to dry out, in 1 tbsp increments.
In another bowl, combine cornstarch with about 2 tbsp of cold water. Mix until it forms a slurry.
Add consnatarch mixture to the pan until it thickens.
Add in your pork and stir until coated. Serve hot!