Melt the butter in a medium skillet over medium-high heat. Add the onions and reduce heat to low. Stir, cover, and cook, stirring every 5-10 minutes and scraping up browned bits that form on the bottom of the pan. Cook for 25-30 minutes, until the onions are deep golden in color and are becoming jammy. Transfer to a bowl and set aside.
Cook the bacon over medium heat in the same pan you used for the onions until crispy. Transfer to a paper towel-lined plate and set aside.
In a large bowl, combine the ground beef with the Worcestershire sauce, salt, pepper, cumin, and hot sauce. Mix until well-incorporated. Shape into 8 patties, about 1-inch thick. Heat the olive oil in a large frying pan and add the burgers, in batches if necessary. Cook to desired doneness, flipping once partway through the cooking time. After flipping, place about 1 tablespoon of the blue cheese on each burger. When the burgers are done cooking, place on a paper towel-lined tray.
Spread 1 tablespoon of the thousand island dressing on the bottom half of each bun, then a lettuce leaf, a slice of tomato, a burger patty, some caramelized onion, bacon, followed by the top buns.