Stir the grated ginger, soy sauce, mirin, sake and sugar together in a small bowl. The ginger juice is the point, so grate it fresh rather than using paste from a jar.
Heat the oil in a large skillet over medium-high. Add the onion and cook 2 to 3 minutes until it softens and picks up a little color at the edges.
Spread the pork out in as close to a single layer as you can. Let it sit 1 minute before stirring, then cook 2 to 3 minutes more until no longer pink.
Pour in the sauce. Toss everything and simmer 2 to 3 minutes until the liquid reduces to a glossy glaze that clings to the pork.
Top with scallions and sesame seeds. Serve over rice with a pile of raw shredded cabbage on the side to cut the richness.