In a skillet, caramelize onions in butter over medium heat until deep golden brown, about 25 minutes. Adjust heat as needed; do not let the onions burn, and stir every 5 minutes.
Remove onions from skillet and set aside.
Deglaze pan with beef broth and sherry; simmer for 3 minutes. Remove from heat and let cool.
In a mixing bowl, combine the ground beef, panko, half the caramelized onions (rough chopped; this helps distribute the caramelized onions evenly in the meatloaf), beef/sherry broth, Worcestershire sauce, thyme, beaten eggs, 1 cup shredded cheese, Kosher salt, and black pepper.
Refrigerate the mixture for 30 minutes. This step helps the proteins bind and will result in a meatloaf that holds its shape.
Shape the mixture into a loaf in a baking dish and top with most of the remaining caramelized onions. Save a little caramelized onion for the final baking step.
Bake the meatloaf at 375°F for 40-50 minutes, or until the internal temperature reads 160 degrees. If the top of the meatloaf is getting too brown, cover it with your ovenproof covering of choice. *Final Baking Step: A few minutes before the meatloaf reaches the internal temperature, top the meatloaf with the remaining cheese mixture and caramelized onions reserved. Remove from oven and let rest for 5 minutes before slicing and serving.