First, melt the butter in your saute pan over medium heat. Dip the top of the bun and the bottom of the bun (on the outside part) into the butter to give it a light coat.

Put mushrooms into the pan, and saute until both sides are golden and mushrooms are cooked. While they are still in the pan, pour in balsamic vinegar and let it reduce until thick and sticky (it takes 4-5 minutes).

Back to the bun - start making your sandwich by placing a slice of thinly cut provolone on the bottom of the bun. When your mushrooms are finished, put them on top of the cheese, leaving all of the glaze in the pan. Next, pile up the baby spinach on top, then the second piece of thin provolone.

Before you top off the bun, use the top bun to soak up all of the balsamic sauce (cut side down), then put on top of the sandwich.

Use the wet paper towel to clean out your pan, then put the sandwich in bottom side down. Place the whole can of tomatoes on top to act as a sandwich press, or you can use a panini maker, or heavy pan. You want it to squish the sandwich flat. Let cook 2-3 minutes, flip with the spatula and cook an additional 2 minutes.

Once golden brown, cut & serve.