Preheat the oven to 400˚F. Line a rimmed baking sheet with parchment paper and set aside.
To make the meatballs, add breadcrumbs and milk to a bowl, stirring to combine. Set aside.
In a cast iron skillet, heat oil until it shimmers over medium-high heat. Add onion and cook until translucent, about 5 minutes, stirring occasionally. Remove from heat.
In a large mixing bowl, add ground beef, breadcrumb mixture, cooked onions, egg, parsley, garlic, Head Country Championship Original Seasoning, and parmesan cheese. Mix together until it just comes together. Be sure not to overmix or the meatballs may become tough.
Form the meat mixture into 12 meatballs (about 2.6-2.7 oz each). Place back in the skillet that you cooked the onions in. Transfer to the oven and bake for 20 to 25 minutes until no longer pink in the center.
Reduce oven temperature to 350˚F.
While the meatballs are baking, stir together tomato puree and Head Country Original BBQ Sauce together in a small bowl. Set aside.
Pour the tomato BBQ sauce over the meatballs, coating each one well with the sauce.
Cut the dinner rolls in half through the middle. Place the bottom half of the rolls on to the prepared, lined baking sheet.
Top rolls with meatballs and a spoonful of sauce. Sprinkle over cheese evenly and place the top half of the rolls on top.
In a small bowl, stir together whisk together melted butter and seasoning. Brush the tops of the sliders with the butter.
Bake for 15 to 20 minutes, until the cheese is melted and rolls are golden on top.
Cut into individual sliders and serve.