Slice the beef into thin slices. Place in a bowl and add 4 tsp. of soy sauce, 1 tbsp. of rice wine, 1 tsp. buckwheat flour and season with freshly ground black pepper.
In a small bowl, mix 3 tbsp. soy sauce, 1 tbsp. water and 2 tsp. buckwheat flour, then set aside.
Heat 1 tsp. oil in a pan on high heat. Add the beef and cook for around 20 seconds letting the beef brown. Next, stir the meat, cooking another 2 minutes and transfer onto a plate.
Add the remaining 1 tsp. of oil to the pan, add the peppers and onions and cook about 4-5 minutes. Place the beef back into the pan, add the prepared sauce and red pepper flakes (optional). Stir fry for about 30-60 seconds on medium heat until slightly thickened.
Serve with rice (not included in nutrition info per serving).
Store in the fridge for up to 4 days.