Move the meatballs around back into the full pan. Season the gravy with salt and pepper and bring to a full boil. Cornstarch needs to boil to activate thickening. After about 3 to 5 minutes, the sauce should begin to thicken.
If the sauce becomes too thick add a bit more beef broth or water. If the sauce is too thin, mix another 1 tbsp of cornstarch into 1 tbsp of cold water and stir to combine. Then, while whisking, slowly drizzle this mixture directly to the sauce to thicken.
Taste and adjust seasonings, if necessary.