In a bowl, mix together soy sauce, sugar, garlic, sesame seeds, sesame oil, pepper and red pepper flakes.
Place marinade in a zipper plastic bag along with the steak and marinade in the refrigerator for 1 hour to 24 hours. I prefer overnight.
Heat grill to a high heat. 350-400 degrees F.
Grill beef for about 5 minutes on each side for a medium finish. Grill longer for more well done meat. (see below for stovetop method)
Slice meat thinly against the grain.
Notes
Alternate Cooking Method for Korean Barbeque:
Slice meat into thin slices.
Place in marinade and squish around to make sure all surfaces of meat are covered. Allow it to sit at room temperature for at least an hour or I like to refrigerate it for 8 hours to overnight.
Heat 1 Tbsp. vegetable oil in a large skillet over medium-high until oil is shimmering.
Remove half of meat from marinade, drain off excess marinade back into the bag.
Cook in hot skillet in a single layer without moving it until it is lightly browned, about a minute.
Can salt if desired.
Toss meat and keep cooking until edges are crisp, about 3 minutes. Transfer to a plate.
Add an addition tablespoon of oil to pan, heat and add remaining meat. Repeat process.