Preheat the oven to 400ºF. Coat a baking sheet with cooking spray and set aside.
Remove stems from mushrooms. Chop the stems very fine and place the mushrooms stem side up on the prepared baking sheet.
Heat 1 teaspoon olive oil in a medium skillet over medium heat. Add the spinach to the hot pan and cook, stirring, until wilted, about 2 minutes. Remove the spinach from the pan and allow to cool. Squeeze out any extra water and chop.
In the same pan, heat the remaining teaspoon olive oil over medium heat. Add the garlic and cook, stirring, until golden, about 2 minutes.
Add the chopped mushrooms stems and continue to cook, stirring occasionally, until soft, about 2 minutes more.
Remove from the heat and transfer to a bowl along with the spinach, breadcrumbs, and cheese. Stir to combine or work it together with your fingers.
Season the mushroom caps with salt. Divide the spinach mixture evenly amongst the mushrooms, rounding the tops. Spray the stuffed mushrooms lightly with cooking spray. Bake 20 minutes or until golden. Eat warm.
Notes
Tips for customizing vegetarian stuffed mushrooms
Leave out the cheese or use a vegan parmesan to make these vegan stuffed mushrooms
Swap the parmesan for crumbled feta or goat cheese
Meat eaters can add crumbled bacon
Use gluten-free bread crumbs to make them gluten free
Swap the spinach for another leafy green such as arugula