Steak Stir-fry

Beef is very expensive in Japan so I don't often cook with it. Sometimes I can find a block of Rump steak at a reasonable price, so it is always a treat. I often just cut it up and fry it if I am in a hurry, but I sometimes like to add some flavour.
500g Rump steak block
2 tbsp olive oil
1 onion
3 green peppers
1 clove garlic, minced
1 tsp ginger, minced
3 stalks spring onion
2 tbsp cornstarch
1/2 cup sake or dry white wine
3 tbsps soy sauce
1 tbsp mirin
Salt and pepper
2 tsp Sesame oil
Slice steak into strips and soak in 1/4 cup of sake. Meanwhile, slice onion and green peppers into thin strips. Chop green onion into 3 cm stalks.
Drain off remaining sake from meat and coat in cornflour. Fry in olive oil on a high heat. Remove from pan once browned. Add onions, green pepper, garlic, ginger, and fry over a medium heat for 3-4 mins. Add remaining sake, soy sauce, 1/2 cup water and salt and pepper. Bring to the boil. Return meat to the pan and add spring onions. Simmer for 2 mins. Add mirin and sesame oil.