Saute sliced onion in oil in a pan for 2 minutes, add thinly sliced beef.
Season with 1 tsp sugar, 1 tbsp cooking wine or cooking sake, 3 tbsp mirin, 3 tbsp soy sauce. Add additional soy sauce and mirin as needed. Let simmer until beef is cooked to your desired doneness, and add pepper to taste
Serve over steamed short or medium grain rice, with a sous vide egg and diced scallions.