In a food processor fitted with a blade, chop the mushrooms in 2-second pulses until they resemble couscous. Alternatively, you can chop the mushrooms by hand.

In a non-stick skillet, over medium-high heat, cook the mushrooms and onions until they are soft and their moisture is gone, about 6 to 8 minutes.  Add the garlic and cook 1 more minute. Set aside to cool.

In a large bowl, combine the beef, mushroom mixture, Worcestershire sauce and egg until blended. Divide mixture into 6 equal portions, form into balls, being careful not to compact the mixture. Gently flatten to between 3/4 inch and 1 inch (2 cm to 2.5 cm). Using your thumb or a spoon, create an indentation in the centre of the burger. Refrigerate until ready to cook.

Slightly salt the burgers on both sides just before cooking. On the stovetop using a skillet over medium-high heat, or over a barbecue grill using direct flame, cook burgers until internal temperature reaches  160°F (71°C), about 5 minutes per side, flipping once. Transfer patties to a platter and allow to rest 5 minutes before placing on bun and garnishing with desired toppings.