1. Preheat oven to 350 deg. In a lrge heavy skillet, heat the olive oil over med heat. Add the onion and cook, stirring, until softened, about 6 min. Add the sun dried tomatoes and spinach and cook until the spinach wilts, about 2 minutes. add the pizza, half the cheese. the chicken broth and basil and cook until the cheese melts, about 2 min. Season the stuffing with salt and pepper and remove from the heat.
2. Using a spoon, scrape the gills from the inside of each mushroom cap and discard. Brush the top of each mushroom cap with olive oil and place upside down on a baking sheet.
3. Divide the stuffing among the mushroom caps; sprinkle with the remaining cheese. Bake until the cheese is bubbly, 20 min. Top with more chopped basil.