Prep: 1. Preheat grill. 2. Slice mushrooms, onion, and mince garlic. Slice cheese. Make: 1. Melt butter in a large skillet over medium-high heat. Add mushrooms, onions, and 1 teaspoon salt. Cook, stirring occasionally, until the mushrooms are tender and starting to release their liquid, about 3 minutes. 2. Reduce heat, add garlic and continue to stir occasionally until mushrooms and onions are golden brown, about 5 more minutes. Place in small bowl and cover with foil. Set aside. 3. Meanwhile, combine ground beef, Worcestershire sauce, 1/2 teaspoon salt, and 1/2 teaspoon pepper in a large bowl. 4. Shape beef into patties and season with remaining salt and pepper. 5. Grill burgers for about 3-4 minutes per side (a little more if you want a well done burger). Place cheese on top of burgers, close lid, and cook for 1 minute longer to let the cheese melt. 6. Toast burgers on grill. 7. Place burgers on toasted buns, and divide the mushroom-onion mixture evenly over the burgers.
You can also do these on the stovetop, just heat a grill pan or skillet to medium-high heat and cook burgers for 3-4 minutes per side. Check out our favorite tips for grilling here - http://blog.gatheredtable.com/2015/06/26/barbecue-basics/