Beef Stir-Fry with Bell Peppers and Sesame Seeds

Beef Stir-Fry with Bell Peppers and Sesame Seeds
Ingredients:
1 lb (450g) beef sirloin or flank steak, thinly sliced
1 red bell pepper, sliced
1 yellow bell pepper, sliced
1 green bell pepper, sliced
1 medium onion, sliced
3 tbsp soy sauce
1 tbsp oyster sauce
1 tbsp cornstarch
1 tbsp sugar
1 tbsp sesame oil
2 tbsp vegetable oil
2 garlic cloves, minced
1 tsp grated ginger