Wipe mushrooms with a damp towel.

In a medium bowl, combine balsamic vinegar, Dijon mustard, olive oil, garlic, rosemary, salt and pepper. Place mushrooms in a large zipper topped bag, cover with mixture, make sure it’s fully saturated, then seal and place in refrigerator. Marinate the mushrooms for at least an hour or up to 24 hours.

At time of cooking, preheat outdoor grill to medium high heat.

While grill preheats, begin making caramelized onions.

Heat a large skillet over medium high heat. Add olive oil to pan followed by sliced onion and reduce heat to low and cook for for about 20 to 30 minutes, or until onions are caramelized. Then deglaze the pan with white wine vinegar and add salt to taste. Remove onions from heat.

Remove mushrooms from marinade and place on grill. Cook for 3 to 4 minutes or until tender and slightly caramelized.

Serve mushrooms on buns with a big helping of caramelized onions on top! Feel free to add any other toppings or condiments of your choosing.