Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
Step
2
Cook pasta as directed on package. Reserve 1/2 cup pasta cooking water. Drain pasta; return to stockpot. Add cream; toss. Set aside.
Step
3
Spray 12-inch sauté pan or skillet with cooking spray; heat over medium heat. Add bacon; cook until crisp. With slotted spoon, remove bacon from pan to plate; set aside.
Step
4
Add onions, salt and pepper to bacon drippings in pan; cook and stir 4 to 7 minutes or until translucent. Add mushrooms; cook 6 to 10 minutes longer or until browned and liquid evaporates. Add spinach and reserved pasta cooking water; cook and stir 1 minute.
Step
5
Pour mushroom mixture over pasta; toss to coat. Pour into baking dish. Top evenly with both cheeses. Cover dish tightly with foil.
Step
6
Bake 20 minutes. Remove foil; top with reserved bacon. Bake uncovered 5 minutes longer.
Prepare and assemble the casserole, without baking, up to a day ahead. Bake at 350°F for 40 minutes. Then, remove foil, add bacon and bake 5 minutes longer.
Lose the bacon for a meat-free twist, or just add bacon to bacon lovers' bowls for a pasta that pleases vegetarians and meat-eaters alike.