Preheat the butter or olive oil in a large skillet over medium-high heat. Add the mushrooms to the HOT skillet and leave undisturbed for 2-3 minutes, before flipping them to brown on the other side for 2 minutes.
Add the minced garlic, crushed red pepper flakes, and spinach to the skillet.
Add a tablespoon of water and cook, stirring constantly for 1-2 minutes, or until the spinach is just wilting. Season with sea salt and freshly cracked pepper, to taste.
Pour into a serving bowl and top with shaved Parmesan.
For a little extra flavor, add a drizzle of balsamic vinegar, if desired.