Preheat the oven to 350 degrees . In a large, heavy skillet, heat the olive oil over medium heat. Add the onion and cook, stirring, until softened, about 6 minutes. Add the sun-dried tomatoes and spinach and cook until the spinach wilts, about 2 minutes. Add the pizza, half of the cheese, the chicken broth and basil and cook until the cheese melts, about 2 minutes. Season the stuffing with salt and pepper and remove from the heat.
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.
Divide the stuffing among the mushroom caps; sprinkle with the remaining cheese. Bake until the cheese is bubbly, 20 minutes. Top with more chopped basil.