Toast sesame seeds in a small pan over medium-high heat for 1-2 minutes. Move them around during the toasting process, so they do not burn. Remove from heat, place seeds in a small bowl and set aside. Once cooled, cover and store until ready to use.
Whisk marinade ingredients together until the sugar has dissolved. Place the flanken ribs into a resealable plastic bags. You might need two or more for this recipe. If you don't have any bags handy, then place ribs into a large glass baking dish. Pour equal portions of marinade mixture over ribs. Make sure they are well coated. Seal bags (or cover with glass dish with plastic wrap), and place into refrigerator for 6-12 hours.
Preheat grill for high heat.
Remove flanken ribs from bags (or glass dish), and place onto grill. Cook for 3-4 minutes per side, depending on thickness. While these do tend to cook quickly, some pieces might be thicker and need an additional 1-2 minutes per side. The same is true for thinner pieces. Reduce cook time and watch for burning.
Once cooked through, place ribs onto a large serving platter and garnish with green onions and toasted sesame seeds. Serve immediately.
Store leftovers in an airtight container in the refrigerator for 4 days after initial cooking.