1. If your beef sirloin is not already thinly sliced, partially freeze it for about 30 minutes to make it easier to slice very thinly against the grain.
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2. In a large mixing bowl, combine the soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, grated Asian pear, and ground black pepper. Whisk the ingredients together until the brown sugar is completely dissolved.
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3. Add the thinly sliced beef sirloin and thinly sliced yellow onion to the marinade mixture in the bowl.
