Take the stems out and scoop the washed baby bella mushrooms. Place on an aluminum foil linked sheet pan.
In a large skillet, add your defrosted frozen spinach, basil pesto and minced garlic. Once this mixture is warmed up add in the cheese and mix again. Set aside to cool off. The mixture has to be cooled down before you pop it into the mushrooms.
Add one spoonful of the spinach mixture to the mushrooms and cook for 10 minutes. If you want less time cook the mushrooms for 8 minutes.