In a large bowl, combine the ground beef, breadcrumbs, egg, milk, chopped onion, minced garlic, Worcestershire sauce, salt, pepper, and thyme. Mix gently until just combined.
Divide the meat mixture evenly among the muffin cups, pressing down lightly.
In a small bowl, whisk together the ketchup, brown sugar, and mustard.
Spoon the glaze over each meatloaf muffin.
Bake for 25–30 minutes, or until the internal temperature reaches 160°F (71°C). Let the muffins rest for 5 minutes before serving.