In a small bowl, combine the soy sauce, mirin, sake, and sugar for the ginger sauce.
Scrape off the outer ginger skin with a knife (or spoon) and cut off any tough parts.
Using a grater (I love this one), grate the ginger, keeping the juice. Collect the grated ginger and squeeze the liquid. You will need a total of 1 Tbsp ginger juice for 2 servings.
Add half of the ginger juice to the sauce bowl. Keep the rest to marinate the pork in a later step. Add as much grated ginger as you like. My family likes a strong ginger taste, so we add about 2-3 tsp for 2 servings.
Next, grate the onion until you get 1 Tbsp of grated onion, including the juice, for 2 servings. Keep the rest of the onion for slicing later.
Add the grated onion with juice to the ginger sauce and mix it all together.
To Prepare the Ingredients
Cut the rest of the onion into thin slices.
If your pork slices are thicker than ⅛ inch (3 mm), cut several slits on the connective tissue (the white area) between the meat and fat. Red meat and fat have different elasticities, and they will shrink and expand at different rates as they cook. These slits will allow the pork slices to stay nice and flat while pan-frying and prevent them from curling up.
Sprinkle the rest of the ginger juice and sake on the pork slices.
When you're ready to cook the pork slices, lightly sprinkle with flour. It prevents the pork from drying up and keeps the juices inside the meat. If your pork slices are paper thin, which is less than ⅛ inch (3 mm) thick, you do not need to dust with flour.
To Cook the Pork
Preheat a large frying pan over medium-high heat. When the pan is hot, add the oil. Then, add the pork slices in a single layer, turning them over once the bottom is golden brown. Cook in batches so the meat sears properly instead of steaming in an overcrowded pan.
When the pork is no longer pink, transfer to a plate. Make sure not to overcook the pork at this stage as we will continue to cook it in the sauce later.
Once you've removed the pork, add more oil to the pan (especially if your pan is not non-stick) and add the onion slices.
Sauté the onion slices on medium heat until golden brown, about 6-8 minutes. When onion slices are tender and translucent, add the pork back to the pan.
Stir the sauce one last time and pour it over the pork. Bring the sauce to a simmer and spoon it over the pork for about 2 minutes. Season with freshly ground pepper to taste (optional).
When the sauce has thickened, the ginger pork is done. Transfer the ginger pork to a serving plate and enjoy.
You can keep the leftovers in an airtight container and store in the refrigerator for up to 3 days or in the freezer for a month.