Mix the sauce: Whisk the soy, mirin, sugar, and water together in a small bowl until the sugar dissolves. Set the sauce aside.
Prep aromatics & veg: Slice the onion and finely dice the garlic. Thinly slice green onions for garnishing.
Pan-fry the pork: Heat 2 tbsp of oil in a hot pan. Add the pork slices and pan-fry until cooked through and starting to brown, about 4–5 minutes.
Add onion & garlic: Toss the sliced onion and diced garlic into the pan with the pork. Cook and stir for around 2 minutes until the onion softens and becomes fragrant.
Pour in the sauce: Pour the prepared sauce over everything and bring to a gentle simmer.
Thicken the sauce: Let it simmer until the sauce reduces and thickens enough to coat the pork and onions - about 2 minutes. Give everything a few stirs so the sauce clings to the meat. Taste sauce and adjust sugar levels if you prefer a sweeter sauce.
Serve: Spoon the pork and sauce over a bowl of steaming rice. Drizzle any extra sauce from the pan over the top and finish with chopped green onion.