Heat a large skillet over medium-high. Add ground beef, granulated garlic, kosher salt, and black pepper.
Cook, stirring occasionally, until beef is browned and no longer pink, about 6–8 minutes.
Stir in diced onion and cook, stirring often, until onions soften and beef crisps in spots, about 4–5 minutes.
Pour in the barbecue sauce and stir to coat evenly. Reduce heat to low and simmer for 5 minutes to thicken and blend flavors.
Keep on low heat if serving immediately, stirring occasionally. Or transfer to a slow cooker on warm until ready.
Spoon the hot filling onto buns and serve right away.