Toast sesame seeds in a small skillet over medium heat until lightly browned, 3 to 5 minutes.
Combine onion, Asian pear, dark soy sauce, light soy sauce, rice wine, sesame oil, ginger, garlic, and black pepper in a blender. Puree into a smooth marinade. Add sesame seeds; blend briefly on low just to mix them in.
Spread a thin layer of marinade in the bottom of a large container. Add a single layer of beef. Cover beef evenly with another thin layer of marinade. Repeat with remaining beef and marinade. Cover container and refrigerate until flavors combine, at least 3 hours and preferably overnight.
Heat a large grill pan or skillet over high heat. Add all the beef and marinade; cook, stirring constantly, until most of the marinade evaporates and the beef starts to brown, 3 to 4 minutes. An instant-read thermometer inserted into a slice should read 140 degrees F (60 degrees C).
A Bartlett pear can be substituted for the Asian pear if desired.
The use of light and dark soy sauces is important to this recipe. They are readily available at any Chinese grocer, and can easily be ordered online. If you wish to use mass-market or Japanese style soy, use a low-sodium variety, or cut back to 1/4 cup total.
Shaoxing wine is readily available at Chinese grocers, as well. Look for a bottle that says Shaoxing or Shao Xing somewhere on it, not necessarily in the name. It has a malty flavor, so grape-based wines will not work. The closest Western equivalent would be sweet vermouth, but it's worth getting the Shaoxing.
For the meat, a carne asada cut works well, as does round or pork loin cut to 1/4 inch. You can use thinner slices of meat, but they won't hold up on the grill, so you'll have to pan-fry. For thicker slices, add 1½ tablespoon white vinegar, and marinate overnight.