One at a time, place the pork slices between 2 sheets of plastic wrap and pound with a meat pounder to ¼-inch thickness. Season both sides with salt and pepper.
In a large sauce pan over medium-high heat, warm the oil. Working in batches, add the pork in a single layer and cook until well browned on both sides, 2 to 4 minutes per batch. Transfer to a plate.
Add the onions to the pan and cook, stirring occasionally, until browned, 7 to 9 minutes. Add the aliños sauce and cook, stirring constantly, about 1 minute.
Add the water and stir to scrape up the browned bits from the pan bottom. Return the pork to the pot and bring to a boil. Reduce the heat to low, partially cover the pot and cook until the pork is very tender and the sauce has thickened, about 1 hour and 15 minutes.
Serve with white rice on the side.