Add honey, soy sauce, ketchup, brown sugar, garlic, ginger and red pepper flakes to a large pan over medium-low heat. Gently whisk ingredients together until smooth. Heat through until small bubble start to form around the edge. You want a simmer, not a boil; about 2-3 minutes.

Pour about 2 tablespoons of cornstarch into a bowl and add 2 tablespoons of the simmering mixture. Whisk to combine until smooth. Whisk the cornstarch mixture into the saucepan. Continue to whisk until completely incorporated. 

Cook over medium-low heat for several minutes or until thickened. Add the cooked meatballs to the sauce and toss to coat. Continue to stir over medium-low heat until meatballs are heated through and completely coated with the sauce.

As an appetizer, serve immediately, or transfer to a slow cooker to keep warm. Or serve as a main dish with rice or pasta. Garnish with sliced green onions and sesame seeds before serving if desired.

Store in a tightly sealed container for several days in the refrigerator.

*Slow Cooker version: Add frozen (or cooked) meatballs to the slow cooker. Whisk remaining ingredients (except cornstarch) in a bowl and pour over the meatballs. Cook on low for about 3 hours. To thicken, add 2 tablespoons of the sauce from the slow cooker to a small bowl and whisk in 2 tablespoons of cornstarch until completely smooth. Pour into the slow cooker and stir to combine. Cover and cook on low for an additional 30 minutes to thicken.