Warm the olive oil in a non-stick skillet over medium heat and then add the mushrooms. Pour the tamari and the balsamic on top then sprinkle with rosemary and a pinch of salt & pepper. Stir everything together and cook for 5-7 minutes, just until mushrooms are tender. Set aside on a plate lined with a paper towel and then wipe out the skillet.

Slice the side of the baguette lengthwise (or grab two pieces of sandwich bread) and layer one piece of fontina cheese inside. Layer the mushrooms inside with the spinach and more fresh rosemary on top, and then finish with another 1/2 (or whole) slice of cheese.

Place the sandwich inside of the skillet and cook on low heat until cheese starts to melt. Serve warm and savor every last cheesy bite!