Heat a large skillet over medium heat. Add bacon and cook, stirring occasionally, until crisp, about 8 minutes. Use a slotted spoon to transfer to a paper towel-lined plate, and set aside to cool. Form beef into eight thin, round patties. Divide bacon and gorgonzola between four of the patties, then top each with one of the remaining patties. Press the edges of each burger closed to seal well. Season burgers all over with salt and pepper.
Prepare a grill for high heat cooking. Grill the burgers until your desired doneness, about 4 minutes on each side for medium (alternately, cook in a large skillet over medium heat, 2 to 3 minutes per side). Top burgers with tomato and onion and serve on buns.
Per Serving:390 calories (160 from fat), 18g total fat, 8g saturated fat, 90mg cholesterol, 880mg sodium, 26g carbohydrates, (4 g dietary fiber, 6g sugar), 34g protein.