Add the frozen meatballs into the slow cooker and add in the cup of water.
Cook on high for 3 hours. This is going to thaw the frozen meatballs and heat the water so that the cornstarch thickens faster.
At 2.5 hours of cook time, mix together the hoisin sauce, soy sauce, sesame oil and cornstarch then, pour that into the meatballs and stir.
Return in 15 minutes to give the meatballs a stir. Then, after the final 15 minutes give them one last stir before turning the crockpot to warm. You're ready to serve, enjoy! Be careful, they will be hot!