Blue Cheese Stuffed Burgers

For 4 burgers:
1 lb ground beef
1 Tbsp. Worchestershire sauce
1/2 tsp. salt
1 tsp. freshly ground black pepper
1/3 to 1/2 C. cumbled blue cheese (2 oz.)
1 medium red onion, sliced crosswise
olive oil
1 C. fresh baby spinach
4 hamburger rolls, split

In a bowl, combine beef, Worchestershire sauce, salt and pepper. On waxed paper, shape into eight thin, 4in. diameter patties. Place 1 Tbsp. of the blue cheese in the center of 4 patties. Top with remaining patties and pinch edges to seal.
Brush onion slices with olive oil and sprinkle with salt.
Place burgers and onions directly over medium-high grill. Grill 5 min. per side or until burger is no longer pink.
Brush cut sides of buns with olive oil and grill cut side down the last minute of grilling.
Serve burgers on buns with grilled onions, spinach, and remaining cheese.