Preheat oven to 350°F (175°C). Lightly grease a 12-cup muffin tin or use muffin liners.
In a large mixing bowl, combine ground beef, breadcrumbs, egg, finely diced onion, minced garlic, 1/4 cup of ketchup, Worcestershire sauce, salt, and pepper.
Gently mix with your hands until just combined, being careful not to overmix.
Divide the meatloaf mixture evenly among the 12 muffin cups, filling each about 3/4 full.
In a small bowl, whisk together the remaining 1/4 cup of ketchup, brown sugar, and apple cider vinegar to make the glaze.
Spoon the glaze evenly over the top of each mini meatloaf.
Bake for 25-30 minutes, or until the internal temperature reaches 160°F (71°C).
Let the mini meatloaves rest in the muffin tin for about 5 minutes before serving.