1Heat oven to 375°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
2Cook and drain pasta as directed on pack- age, using minimum cook time. Meanwhile, in 8-inch nonstick skillet, cook mushrooms and onion over medium heat about 5 minutes, stir-ring frequently, until softened.
3In large bowl, stir together cooked pasta and spinach. Stir in mushroom mixture. Spread in baking dish. In another bowl, stir remaining ingredients until blended. Pour over pasta mixture.
4Bake uncovered 35 to 40 minutes or until top is golden brown. Let stand 5 minutes before serving.