In a medium skillet, combine the ground turkey, onions, spinach, fire roasted tomatoes and garlic. Season the turkey mixture with salt, pepper and Italian seasoning and cook over medium-high heat until turkey is no longer pink.
Meanwhile, preheat oven to 425 degrees. Remove stems from mushrooms and scrape out the gills as best as you can. Place "gill" side up on a lightly sprayed baking sheet.
Once the turkey is no longer pink, drain turkey mixture onto a paper-towel lined plate to remove any excess liquids.
Place turkey mixture back into the skillet and add the cheese and marinara sauce. Mix well until the cheese has melted.
Stuff the mushrooms with the turkey mixture and bake for about 20 minutes. Add extra cheese on top, if desired, and place back in oven for about 5 minutes or until the cheese has melted.