Preheat oven to 375. Meanwhile, brown the ground chicken. Add oregano, salt and pepper. Add fresh garlic and fresh spinach. Sauté about 5 minutes, stirring to combine. Remove from heat and set aside.
Dab the mushroom caps with a web paper towel to clean. Remove the stem and the "guts" with a spoon. Place on baking sheet or baking stone.
Divide the tomato sauce evenly in the bottom of the mushroom caps. Add the chicken mixture. Top with low-fat mozzarella cheese.