We love the idea of putting the cheese inside the burger, rather than on top of it. Make this with chuck alone for a juicier burger, although if you opt for sirloin alone, the cheese-mushroom filling will provide some moisture.
1 oz. dried mushrooms, see note
1/2 cup very hot water
2 tbsp. Worcestershire sauce
2 tsp. dried marjoram
4 oz. Gorgonzola cheese, crumbled
4 scallions, finely sliced
Vegetable oil cooking spray
Put the mushrooms in a small bowl and add about 1/2 cup very hot water. Set aside for about 20 minutes for the mushrooms to hydrate.
Combine the beef, Worcestershire sauce, marjoram, and pepper in a bowl. Set aside.
Squeeze the mushrooms dry and discard the soaking liquid or reserve it for another use. Mince the mushrooms and combine the mushrooms with cheese, scallions, and salt. Divide the cheese mixture into 6 equal portions, flattening each into a thick patty.
Divide the meat into 12 portions and flatten each into a patty. Set a cheese patty on 6 of the beef patties and top with the remaining beef patties. Press the edges of the patties closed to seal tightly and hold in the juices during grilling. Refrigerate until ready to grill.
Prepare a charcoal or gas grill. Lightly spray the grill rack with vegetable oil cooking spray. The coals should be moderately hot to hot.
Grill the burgers for about 5 minutes. Turn and grill for 4 or 5 minutes longer for medium-well burgers. Use a large spatula when turning the burgers, taking care that they do not split.