Preheat your smoker to 250 degrees Fahrenheit.
In a mixing bowl combine ground beef, pork, Italian panko crumbs, yellow onion, milk, Parmesan cheese, salt, eggs, Beef seasoning, pepper, and garlic powder. Mix gently until combined, don't over mix.
Place the bowl in the freezer for about 15 minutes to help thicken the meatball mix. This helps you easily form the meatballs.
Take 9 sticks of mozzarella cheese and cut each one of them up into fifths.
Then take a medium size cookie scoop and scoop the meat mixture out of the bowl, form up a ball in your hand, press your thumb into the middle of the meatball and place in a piece of string cheese in the cavity. Wrap the meat over cheese to make sure that your cheese does not ooze out when cooking it.
Place all your stuffed meatballs on an oiled wire rack and place the balls in your smoker at 250 degrees Fahrenheit and smoke for 35-40 minutes.
Meanwhile a small bowl mix together the BBQ sauce.
After 35-40 minutes of smoking time brush prepared BBQ sauce on the meatballs and cook for an additional 10 minutes or until internal temp is 165 degrees Fahrenheit.
Remove from smoker and serve immediately.