Preheat oven to 350 degrees. Put your mushroom caps on a foil lined cookie sheet for easy cleanup.Mix the mashed potatoes with the spinach and spoon into the mushroom caps.Mix the breadcrumbs with the melted buttery spread, garlic salt and pepper and pile on top of each mushroom.Bake at 350 for about 20 minutes. Keep an eye on them so the breadcrumbs don’t get too brown.Serve immediately.