Heat a large cast iron skillet or pan over medium heat and melt the butter.
Add the sliced mushrooms to the skillet and sauté for about 5 minutes, until they start to soften and release their moisture.
Add the minced garlic to the skillet and sauté for another 1 minute, until fragrant.
Add the fresh spinach to the skillet and cook, stirring occasionally, until the spinach wilts down, about 3 minutes.
Season the mixture with salt, pepper, and a pinch of red pepper flakes, if desired. Adjust the seasonings to your taste.
Cook for an additional 1 minute to allow the flavors to meld together.
Remove from heat and transfer the mushroom and spinach mixture to a serving dish. Garnish with fresh parsley, if desired, and serve immediately.