Preheat oven to 425ºF.
In a bowl, whisk together the vinegar, salt, pepper, garlic and oil.
Pour over the mushroom caps and make sure they are coated evenly.
Place them on a baking sheet, rounded side down.
Let sit for about 15 minutes. Don’t rinse the bowl, because you’ll use any oil and vinegar residue to dress the spinach.
Place baking sheet in the oven and roast for 15-20 minutes, until cooked through.
Remove baking sheet from oven and turn mushroom over.
Place one slice of cheese on each mushroom. Continue baking until the cheese melts.
In a medium bowl, add spinach to the bowl used to marinate the mushrooms. Toss in the residual oil and vinegar to lightly dress.
Place a mound of spinach on the bottom half of each whole wheat bun and place the mushrooms, rounded side up, on top of the spinach.
Top with your choice of toppings (i.e. pickled onions, hummus, avocado).
Serve and enjoy!