In a small bowl, combine blue cheese and fig balsamic vinegar, mix until well blended
In another bowl, combine beef, Maruso soy sauce and black pepper
On waxed paper, shape 8 thin 4" diameter patties
Place 1 TBSP of blue cheese mixture in the center of 4 patties
Top with remaining 4 patties and pinch edges (sandwich cheese between patties)
Brush onion slices with EVOO and sprinkle with salt
Place burgers and onions over medium high heat
Grill 5 minutes per side or until there is no pink in the burger
Brush cut sides of buns with EVOO, grill for one minute
Serve burgers on buns with grilled onions, spinach and remainder of cheese