Active Time: 30 minutes
Total Time: 1 hour
For the Mushroom Stuffing:
8 ounces mixed mushrooms, sliced
8 ounces baby Portobello mushrooms, sliced
2 cloves garlic, minced
2 tablespoons olive oil
1 1/2 tablespoons sherry vinegar
1/2 teaspoon fresh-squeezed lemon juice
3 sprigs fresh thyme
Kosher salt and black pepper
For the Burgers:
1 1/2 pounds ground beef
1 1/2 tablespoons grated onion
1 tablespoon steak sauce
1 teaspoon garlic powder
4 slices of Swiss cheese, plus (4) 1 ½” cubes Swiss cheese
1/4 cup cracked black pepper for crusting
4 hamburger buns
Toppings, as desired
Adjust oven rack to lower-middle position and preheat oven to 400°F.
Place mushrooms garlic, olive oil, vinegar, lemon juice and thyme in a 13″ x 9″ baking pan. Season with salt and pepper and toss to combine. Transfer to the oven to cook until liquid is absorbed and mushrooms are tender, stirring halfway through, about 25 minutes total. Remove from oven. Discard thyme, scoop mushrooms onto a cutting board and finely chop mushrooms.
Light one chimney full of charcoal. When all the charcoal is lit and covered with gray ash, pour out and spread the coals evenly over half of coal grate. Alternatively, set half the burners of a gas grill to high heat. Set cooking grate in place, cover gill and allow to preheat for 5 minutes. Clean and oil the grilling grate.
Place ground beef, onion, steak sauce and garlic powder in a large bowl. Season with salt and pepper and thoroughly combine using hands. Do not over-mix.
Divide meat into four equal portions. Fill each patty with a cube of Swiss cheese and 1 1/2 tablespoons of mushroom filling, creating burgers with thick, flat sides.
Place 1/4 cup pepper on a plate. Roll burgers through the pepper along the outsides to crust the rims.
Place burgers directly over hot coals, cover with vents open, and cook, turning occasionally, until well charred and center of burgers register 110°F on an instant read thermometer, about 5 minutes. Place cheese on top of burgers and continue to cook until cheese is melted and burgers register 125°F for medium rare or 135°F for medium, 1 to 2 minutes longer. Transfer burgers to a large plate.
Toast buns over center of grill until golden brown and warmed through. Top burgers as desired, place in buns, and serve.