Preheat oven to 425°. Evenly drizzle both sides of mushrooms with Olive Oil, then season, if desired, with salt and black pepper. On cookie sheet, arrange mushrooms, gill-side-up; set aside.
In 10-inch nonstick skillet, brown sausage; drain, if desired. Add onion and cook, stirring occasionally, 3 minutes or until tender; cool slightly.
In medium bowl, combine sausage mixture, 1/2 cup Sauce, bread, 3 tablespoons cheese and pine nuts. Evenly spoon sausage mixture onto prepared mushrooms, then sprinkle with remaining cheese. Bake 15 minutes or until mushrooms are tender.
Meanwhile, in 5-quart saucepot, bring spinach and 1/4 cup water to a boil over high heat. Reduce heat to low and cook covered, stirring occasionally, 2 minutes or until spinach is wilted. Season, if desired, with salt and black pepper. With slotted spoon, evenly arrange spinach on plates, then top with mushrooms. Serve with remaining Sauce, heated.
TIP: For VEGETARIAN STUFFED MUSHROOMS, omit sausage and cook chopped mushroom stems with onion.
GREAT VARIATION: To make APPETIZER STUFFED MUSHROOMS, prepare filling as above and stuff into white mushroom caps. Bake 10 minutes or until tender.