Let's cook.
Thinly slice the pork loin against the grain into bite-sized pieces. In a medium bowl, combine the soy sauce, mirin, sake, grated ginger, and sugar. Add the sliced pork to the marinade, ensuring all pieces are coated. Let it marinate for at least 10 minutes at room temperature.
Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the thinly sliced onion to the hot skillet and stir-fry for 2-3 minutes until softened and slightly translucent.
Push the onions to one side of the skillet. Add the marinated pork in a single layer to the empty side of the skillet. Cook for 2-3 minutes without stirring to allow the pork to sear and brown.
Stir the pork and onions together, continuing to cook for another 3-5 minutes, or until the pork is cooked through and the sauce has thickened slightly.
Serve immediately with steamed rice and a side of vegetables, if desired.
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