Slice the onion thinly. Heat the olive oil in a large skillet over medium-high heat. Add the onion and sugar and reduce heat to medium low. Sauté the onions, stirring frequently, until they are golden brown and well caramelized – about 15 minutes. Remove from heat.
When onion has cooled, place it in a small food processor and pulse to a coarse puree.
In a medium bowl, combine the ground beef, Worcestershire sauce, fresh herbs, black pepper and the onion. Mix well.
Divide the meat into 12 equal portions.
Flatten each of the portions into patties – about 1/2″ thick (do not make them too thin).
Place 1 heaping Tbsp. of blue cheese in the center of 6 patties.
Top each with one of the remaining 6 patties and pinch the edges to seal.
Refrigerate the patties for 1/2 hour.
Grill the burgers until they are cooked to your liking.
Serve burgers on toasted buns with avocado, tomato, arugula or lettuce, and your other favorite burger condiments.