Prep your mushrooms by brushing clean, removing the stems and gently scraping out the dark/brown part to make room for stuffing. Place on a grill heated to 400 F and grill each side for 3-4 minutes, until they are browned.
Meanwhile, add olive oil to pan and add chopped garlic and spinach. Sautee until the spinach is wilted.
Take the mushrooms off the grill. Take a paper towel and dab each of them to remove the liquid.
Spoon pizza sauce into mushroom cups. Next, layer sauteed spinach and top with feta cheese.