Preheat oven to 425° F. Roughly chop mushrooms and toss in olive oil. Spread in an even layer on a sheet pan and roast in the oven for about 20 minutes, until soft. Drain off excess liquid.
Place mushrooms in food processor and process until it reaches the consistency of ground beef.
Place ground beef, mushroom mixture, egg, Worcestershire sauce, salt, and pepper in a large bowl. Mix with a spatula until well combined.
Divide beef mixture into 6 equal parts and form into patties. Cook on grill to desired temperature. Serve with choice of bread or lettuce and add desired toppings.
Notes
What you pair with this patty will have an impact on total sodium and carbohydrate amount. Be sure to take this into account, especially if you're watching your blood pressure or blood sugar.