Place spinach in a heavy saucepan with 2 TBS water. Cook over medium heat for 2 minutes or until wilted. Cool, drain and squeeze spinach to remove any remaining water. Finely chop and set aside.
Remove mushroom stems and finely chop.
Heat olive oil in skillet over medium heat. Saute garlic and chopped mushroom stems for 5 minutes or until lightly browned. Stir in spinach and oregano. Saute for 1 minute, stir in lemon juice. Remove from heat.
Fill mushroom caps with spinach mixture. Top with bread crumbs.
Spray baking pan with olive oil spray. Place mushrooms on pan. Bake for 12 minutes or until topping is light gold in color. Serve hot.