Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin tin.
In a large bowl, combine the ground beef, egg, breadcrumbs, chopped onion, milk, garlic powder, salt, and black pepper. Mix gently just until evenly combined, taking care not to pack the meat mixture too firmly.
Divide the mixture among the 8 prepared muffin cups. Press it in lightly, leaving the tops rounded rather than tightly packed.
In a small bowl, whisk together the yellow mustard, honey, apple cider vinegar, and Worcestershire sauce until smooth. Spoon about half of the glaze over the meatloaf muffins.
Bake for 20 minutes. Spoon the remaining glaze over the tops, then bake for about 10 more minutes, until the centers reach 160°F.
Let the muffins rest in the tin for 5 minutes before carefully lifting them out and serving.