Remix the soy sauce mixture and add it to the skillet. Reduce the heat to medium and stir-fry until the beef is well-coated and the sauce has thickened, for about 2 minutes.
Turn the heat off. Spoon the meat and pan juices onto plates or bowls. Top with the green onions and sesame seeds, drizzle with the sesame oil, and serve.
¼ cup green onions, 1 teaspoon sesame seeds, 1 tablespoon sesame oil
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Notes
Saltiness and sweetness preferences are highly personal. Adjust to taste.
To make this recipe as easy as can be, I sometimes use pre-sliced top sirloin (sold in trays and marked as "beef for stir fry"). Of course, it's fine to buy top sirloin steaks and slice them yourself.
Top sirloin is appropriate for this recipe because it's lean, and while it's not super tender, it's also not overly tough, so you can get away with not marinating it. Do make sure, however, that you're getting top sirloin. Bottom sirloin is tough and won't work in this recipe.
You can keep the leftovers in a sealed container in the fridge for 3-4 days. Reheat them covered in the microwave at 50% power. You can also freeze the cooled leftovers for up to three months.