Heat the butter in a skillet over medium heat. Add the sliced onions and cook slowly for 15 minutes until they turn golden and soft. Set them aside to cool.
In a mixing bowl, combine the ground beef, breadcrumbs, egg, minced garlic, Worcestershire sauce, salt, and pepper. Mix until just blended.
Place half the meat mixture into a loaf pan and press it into an even layer. Spoon the caramelized onions down the center, then scatter the blue cheese over the onions. Cover with the remaining meat mixture and seal the edges.
Bake at 375°F for 45 minutes until the internal temperature reaches 160°F. Let the meatloaf rest for 10 minutes before slicing.