Clean mushrooms, pat dry then gently remove the stems from the caps—be careful not to break the caps! Set caps aside and chop stems into small pieces.
Melt butter in a skillet over medium-high heat. Add minced garlic, stir for approximately 1 minute then add mushroom stems and chopped spinach. Sauté for approximately 5 more minutes.
Place a colander in the sink and carefully pour the spinach and mushroom mixture into it. Use a wooden spoon to press the spinach against the side of the colander to squeeze as much liquid out as possible.
Return the spinach and mushroom mixture to the skillet over medium-high heat and cook for just a few seconds to remove any remaining moisture. Stir in the heavy cream then heat until just boiling and reduce the liquid by half.
Remove from heat and add Parmesan cheese, pepper and salt. Stir until combined.
Pre-heat oven to 400 degrees. Lightly brush mushroom caps with olive oil and place on a jelly roll pan or large rimmed cookie sheet.
Spoon the spinach mixture into each cap, mounding slightly. Bake for 30 minutes.
Remove stuffed mushrooms from the oven, dust with a little more Parmesan cheese and serve hot! Enjoy!
If you are vegan, you can experiment substituting coconut or almond milk for the cream and replace the cheese with nutritional yeast.