1. In a medium mixing bowl, use clean hands to combine sirloin, black pepper, garlic powder, and worcestershire sauce. Separate beef into 8 equal sized portions.
2. Press down slightly onto 4 balls to form rough patties and place gorgonzola cheese into the center of each. Top each with additional ball of sirloin and form into patties, being sure to seal the edges all the way around to prevent cheese from oozing out during cooking.
3. Preheat outdoor grill or indoor grill pan. Just before grilling, sprinkle kosher salt onto both sides of each burger patty. Grill burger on each side for approximately 3 minutes for medium rare, 5 minutes for medium, or until desired doneness is achieved.
4. If desired, butter inside of buns and lightly toast on grill for 1-2 minutes.
5. Top burgers with cranberry mustard and romaine lettuce leaves and serve warm.
FOR THE CRANBERRY MUSTARD:
1. Put 1 cup of water and the cranberries in a medium sized saucepan and bring to a boil. Reduce the heat and simmer until the cranberries have burst (about five minutes), then add the sugar and honey and stir until dissolved. Add the vinegar and mustard and stir to combine. Season to taste.
2. Process in a food processor or high speed blender using the pulse button (if you want it really smooth, you can then press the mixture through a nylon strainer, but I leave it chunky). Put in a sterilized jar, then cover and seal with a vinegar-proof lid. Let cool, and keep in the refrigerator for up to two weeks.
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