Combine the sauce ingredients for the Korean BBQ sauce in a medium size bowl and whisk together. Heat a large skillet over medium-high heat and add in the sauce. Carefully add in the frozen/fully cooked meatballs to the sauce. Stir to coat and bring mixture to a hefty simmer.
Cook the meatballs in the sauce, stirring often, until the meatballs are heated through and the sauce has reduced slightly. This should take about 15 minutes.
Combine 2 tsp cornstarch with 2 tsp of cold water in a small dish until smooth in consistency. Pour this cornstarch mixture into the sauce and meatballs and stir until the sauce thickens.
Garnish the meatballs with sliced green onions and sesame seeds. Enjoy!