Mix soy sauce and sugar together in a microwave-safe bowl and microwave on high for 2 minutes. Stir in onion, sesame seeds, sesame oil, garlic, and ginger.
Slice pork into 1/4-inch cutlets and place in a marinating pan or dish, spreading out evenly. Pour the marinade over the pork and refrigerate for at least 4 to 5 hours; overnight is better if you cut them thicker.
Preheat the oven to 350 degrees F (175 degrees C). Spread pork evenly on a nonstick, oven-proof pan. Reserve the marinade.
Bake in the preheated oven until an instant-read thermometer inserted into the pork reaches 160 degrees F (71 degrees C), 15 to 20 minutes. Remove from the oven.
Microwave remaining marinade on high for 2 minutes, or until it begins to boil. Pour over the pork.