Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper or lightly grease a muffin tin.
In a large bowl, combine the ground beef, breadcrumbs, milk, egg, onion, garlic, Worcestershire sauce, salt, pepper, and thyme. Mix gently until just combined.
Divide the mixture into 4 to 6 portions and shape into mini loaves. Place them on the prepared baking sheet or into the muffin tin.
In a small bowl, stir together the ketchup, brown sugar, and apple cider vinegar. Spoon or brush the glaze over each mini meatloaf.
Bake for 20 to 25 minutes, or until the internal temperature reaches 160°F (71°C).
Let rest for 5 minutes before serving.