In a large bowl, mix ground beef, minced onion, stuffing mix, milk, eggs, salt, and pepper until combined.
Shape the mixture into a loaf and place it in the crockpot.
In a small bowl, whisk together ketchup, brown sugar, minced onion, mustard, vinegar, and black pepper to make the glaze.
Spread half of the glaze over the meatloaf.
Cover and cook on low for 6 hours or high for 3 hours.
In the last 30 minutes, spread the remaining glaze over the meatloaf.
Once done, carefully remove from the crockpot and let it rest for 5-10 minutes before slicing.
Serve warm with your favorite sides.