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 a baking dish

Bake for about 15 minutes, or until mushrooms are tender