Add the soy sauce, sugars, mirin, garlic, sesame oil, and pepper to a large mixing bowl. Whisk together until evenly combined.
5 tbsp soy sauce, 2 tbsp brown sugar, 1/2 tbsp granulated sugar, 2 1/2 tbsp mirin, 4-5 garlic cloves, 1 tbsp sesame oil, pepper
Add the steak and white onion to the marinade, and stir together really well. Cover the bowl with a dish towel, and let it rest for 30 minutes while the meat marinates.
1 1/2 lbs thinly sliced ribeye steak, 1 medium white onion
Add the oil to a large cast iron skillet set over medium high heat. Once the oil's hot add the marinated meat and onions to the skillet.
2 tbsp canola oil
Cook the meat for 5 minutes, stirring occasionally, until it's reached your preferred done-ness.
Serve warm with steamed white rice, garnished with the green onions and sesame seeds.
thinly sliced green onions, toasted sesame seeds