Combine mirin, sake, soy sauce, grated ginger, garlic and white pepper. Add to pork and marinate 30 minutes or more.
Heat some oil in a wok (about 1-2 Tbls.). Add sliced onions and quickly stir fry until onions begin to soften, Remove from wok and set aside.
Add a bit more oil to the wok and add pork (drained from marinade), stir fry until browned but not overcooked. Cook in batches as necessary so you don’t overcrowd the pork and end up steaming it. You want to try to get a little color on the pork. Before pork is completely cooked, add onions back into the wok.
Add sauce and continue to stir fry a couple more minutes to combine flavors and finish cooking the pork.
Serve on top of rice. Garnish with sliced green onions and sprinkle of sesame seeds.