Thinly slice the onion, cut the king oyster mushroom in half lengthwise and then into six pieces, and slice the red bell pepper into 5mm strips. Cut the beef into bite-sized pieces, season with a pinch of salt and pepper, and lightly coat with flour using a sieve.

Melt 1 tbsp of butter in a frying pan, add the beef, and stir-fry until the color changes. Remove from heat, add red wine, and swirl the pan to evaporate the alcohol with residual heat. Remove the beef and set aside.

Return the frying pan to the heat, melt another 1 tbsp of butter, and add the onion. Stir-fry until softened, then add the king oyster mushroom and red bell pepper in sequence, stirring after each addition. Sprinkle 1/2 tbsp of flour and mix well until no longer powdery.