1In a medium bowl combine beef, Worcestershire sauce, and black pepper. On waxed paper shape beef mixture into 8 thin 4-inch-diameter patties. Place 1 Tbsp. of the blue cheese in the center of 4 of the patties. Top with remaining 4 patties; pinch edges to seal.
2Brush onion slices with olive oil; sprinkle with salt.
3For a charcoal or gas grill, grill burgers and onions on the rack of a covered grill directly over medium-high heat for 5 minutes per side or until no pink remains in burger. Brush cut sides of buns with olive oil. Grill, cut sides down, the last minute of grilling burgers.
4Serve burgers on buns with grilled onions, spinach, and remaining cheese.