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 coconut oil in a pan on high heat. Add the beef and cook letting the beef brown. Transfer onto a plate.
Add the remaining 1 tsp of coconut oil to the pan, add the peppers and onions and cook about 4-5 minutes. Place the beef back in the pan, add the prepared sauce and red pepper flakes (optional). Stir fry for about 30-60 seconds on medium heat until the sauce thickens a little.
Serve with rice.