Fire up your grill for direct medium-high heat cooking. You can also use an outdoor or indoor griddle or a large skillet on a stove.
Place the meatloaf into large bowl. Using your hands, break it into large pieces.
Add the egg, Worcestershire sauce. thyme and pepper. Combine the ingredients into the meatloaf mix.
Divide the meat into 4 equally-sized portions and shape into balls. Don't pack the meat, just loosely form into balls.
Place the meat balls onto the grill. Smash down with a spatula or grill press to form thin patties. Season well with salt.
Grill the burgers for 2-3 minutes per side. Keep the burger press on top of the burgers as they cook.
Slather one side of the patties with the glaze. Flip and grill anther minute.
Slather top side of the patties with more glaze. Flip and grill another minute or until they reach your desired doneness.
Transfer cooked patties to bun bottoms. If you have more glaze, divide it between the patties.
Top with onion and iceberg and bun tops and serve.