Rinse pork liver under running water for 10 minutes; Fill a large glass bowl with water; Stir 1 cup of white vinegar into the water; Add the pork liver; Leave it for an hour.
Rinse, drain and peel ginger; Slice a small part of the ginger; Grate the remaining with a microplane grater over a bowl.
Put a strainer over a bowl; Line the strainer with a piece of cheesecloth, draping the corners over the sides; Spoon the grated ginger into the center of the cheesecloth; Gather up the 4 corners and twist to squeeze the grated ginger for juice for later use.
Remove the pork liver; Cut off the thin membrane encasing the liver with a sharp paring knife; Trim any tough bits of connective tissue, fat and vein off with the knife; Rinse the pork liver under running water again; Slice it; Put it in a mixing bowl; Add seasoning (1) and the ginger juice from the step 3; Mix; Leave it for 45 minutes.
Rinse spinach under running water; Drain it; Cut off any roots; Discard any wilted leaves; Soak it in water for an hour; Remove it; Rinse it under running water again; Drain it; Cut it into sections.
Fill a deep wok with water; Bring to the boil over a high heat; Blanch the spinach for 1 minute; Immediately remove it and soak it in a bowl of ice water briefly; Remove and drain it; Turn off the heat.
Fill a stockpot with 2.5L water; Add the ginger slices; Bring to the boil over a high heat; Add the pork liver, the spinach and 3 tbsp oil; Bring to the boil over a high heat again; Turn to a medium heat; Cook over a medium heat for 15 minutes; Stir seasoning (2) into the soup; Turn off the heat; Arrange the soup into serving bowls.