Place mushrooms in the bowl of a food processor and pulse on and off until mushrooms are finely chopped but not powdery.
Heat olive oil in a 10-inch, non-stick skillet over medium-high heat. Add mushrooms and onions. Cook and stir until mushrooms are tender and start to brown a bit, about 5 minutes. They should appear dry and crumbly, not wet. Remove from heat and let cool.
In a large bowl combine beef, mushrooms, parsley and burger seasoning. Mix using your hands until all ingredients are well blended. Shape mixture into 4 patties. Cover with plastic wrap and refrigerate for 1 hour.
Preheat grill to medium-high heat and lightly oil grill racks. Grill burgers for about 5 minutes per side, or until cooked to your desired degree of doneness. During the last minute of cooking time, baste with barbecue sauce, if using. Serve immediately with buns or without, topped with your favourite burger garnishes.