Preheat the oven to 400 degrees
Wash and dry mushrooms
In a skillet, add the oil and turn burner on medium heat
Remove the stems from the mushrooms, coarsely chop them and place in skillet
Add the spinach and cook for about 1 minute, or until mushrooms are tender and spinach is wilted
Add the garlic salt and cook for about 30 seconds longer
In a medium sized bowl, mix the rest of the ingredients
Add the spinach mixture to the bowl and fold in
Spray an 8x10 baking dish
Fill mushrooms with spinach mixture and set them bottom side down into baking dish
Bake for about 15 minutes, or until mushrooms are tender