Heat olive oil in a large skillet over medium heat.

Add mushrooms, rosemary and vinegar. Cook until mushrooms have softened, about 8-10 minutes. Remove mushrooms and set aside.

Add another teaspoon of olive oil to the same skillet. Add onion to the pan. Heat onions until soft and caramel in color. This should take about 10-15 minutes.

At the 10 minute mark, add your red peppers and continue to cook just until the mixture is heated through.

Spray a grill pan with non-stick spray and heat on medium low. Add mushrooms, arranging them into two “piles”. (Reserve remaining mushrooms)

Top each pile with onion mixture and 2 oz of cheese. Place a deep skillet cover over the piles and allow cheese to melt.

Meanwhile toast your hamburger buns.

In a small bowl, combine mayo, garlic and salt. Stir. Place about a 1/2 tablespoon of mayo mixture on each half of your bun.

Using a spatula, carefully lift up mushroom mixture and place on the bottom of two buns.

Repeat the process with remaining mushrooms, onions and mayo.

If space allows, heat all your mushrooms at the same time. Alternatively, divide mushroom and onion mixture on the bottoms of 4 buns.

Top each bun with cheese and broil in a preheated oven until cheese has melted.