Place the frozen burger directly on a preheated grill (350°F on indirect heat) or skillet (medium heat).
Wait for the juices to rise to the top without moving the burger around and do NOT press on it with a spatula, approximately 4 to 6 minutes. Browning is a good thing, it helps to develop natural flavors!
Flip the BUBBA burger and cook for another 4 to 6 minutes or until your desired doneness.
Allow the burger to rest on a plate. In the same skillet, prepare the mushroom gravy.
Melt the butter and then add the mushrooms and onion over medium high heat. Saute until the pan is dry, approximately 4-5 minutes. Cook 1-2 minutes longer until the vegetables start to brown.
Stir in the water, Worcestershire sauce, brown sugar, mustard, thyme and water. Season to your liking with coarse kosher salt and freshly ground black pepper.
Continue to cook until the sugar dissolves. There should be enough to coat the mushrooms, but it is not a sauce or traditional liquid gravy. Remove the mushrooms from the skillet and return the burgers to the hot skillet.
Place two slices of the Swiss cheese over each burger, then top with a lid to let the steam and heat gently melt the cheese.
Assemble the burger by placing the cheesy burger on the bottom. Divide the mushroom topping evenly.
Enjoy open-faced or top with the other half of the bun.
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