Fry the burgers over medium high heat until cooked, about 4 minutes each side; set aside.
put about 1 tablespoon of butter in the pan and add thinly sliced onion sprinkle with about 1/2 Worcestershire sauce and sugar and a pinch or so of salt. Sauté until caramelized. about 5-10 minutes Set aside.
Add some more butter to the pan place the mushrooms cap side down and sprinkle with remaining Worcestershire sauce cook until browned and flip to cook the other side. about 3-5 minutes each side.
When mushrooms are done you may begin assembly place one mushroom cap side down on serving ( microwave safe) plate. Set burger on top of mushroom. cover burger with slices of cheese and the barbecue sauce.
While assembling the burger sauté the tomato in same pan about 3 minutes each side. when cooked on both sides carefully place on top of cheese and burger. Top with the caramelized onions and finally the second mushroom bun cap side up this time.
Put burger in the microwave and heat for 45-60 seconds to reheat burger and melt the cheeses slightly.
Note: Unlike other burgers, this will need a knife and fork. I'm not certain until the nutritional stats are up but I suspect this recipe might be good for the low carbers.