2 pounds sirloin steak tipstrimmed (each about 1-inch-thickness)
In a small bowl, whisk the soy sauce, brown sugar, scallion, garlic, red pepper flakes, sesame oil, and olive oil.
Pour into a gallon-sized sealable plastic bag, add the steak tips, and seal the bag, pressing as much air out as you can. Turn the bag a few times to move the marinade around, coating the steak. Refrigerate for at least 1 hour or up to 24 hours, flipping the bag halfway through to make sure the steak marinates evenly.
Preheat your gas grill with all burners on high and close the lid (15 minutes). When the grill is hot, lower all of the burners to medium-high. Remove the steak tips from the bag, discard the marinade, and arrange on the grill. Cover and cook until the undersides of the tips have deep grill marks, 8-10 minutes. Flip and cook on the other side until browned and the steak registers 130 degrees on instant-read thermometer (for medium doneness), about 8 more minutes.
The steak should be slightly less done than you want (since it will continue to cook off of the grill).
Transfer to a plate, tent with foil, and let the steak rest for 5 minutes before slicing very thin on the bias. Serve.