Finely chop onion and mushroom caps. Mix all ingredients together. Allow to set in fridge for 1 hour for flavors to absorb. Pour olive in skillet on medium low heat. Allow oil to heat up for a min. Form mushroom mixture into patties and fry on both sides just until slightly crispy about 3 mins per side. Take care in turning over as they tend to stick a bit. Serve on favorite bun with your choice of toppings. Enjoy.
For a Asian flavor, substitute teriyaki sauce for the A1 sauce and mozzarella cheese for the pepper jack.