In a large bowl mix together the balsamic vinegar, rosemary, tamari & olive oil. Then add the mushrooms and coat well with the marinade on both sides. Let sit for about 20min.
Cook the mushrooms on medium heat in a skillet or on the grill for about 5 - 8 min on each side. When the mushrooms are finished cooking, place them on the buns. Add all of your favorite toppings and enjoy!