Preheat oven to 425 degrees. 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.
Brown sausage in 10-inch nonstick skillet; drain, if desired. Add onion and cook, stirring occasionally, 3 minutes or until tender; cool slightly.
Combine sausage mixture, 1/2 cup Sauce, bread, 3 tablespoons cheese and pine nuts in medium bowl. Evenly spoon sausage mixture onto prepared mushrooms, then sprinkle with remaining cheese. Bake 15 minutes or until mushrooms are tender.
Meanwhile, bring spinach and 1/4 cup water to a boil in 5-quart saucepot 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. Evenly arrange spinach on plates with slotted spoon, 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.