Heat oven to 400°F Cook and drain fettuccine as directed on package.
Meanwhile, in 10-inch skillet, melt butter over medium heat. Add onion; cook 3 to 4 minutes, stirring frequently, until tender. Stir in garlic, salt and both cans of tomatoes. Cook 5 minutes, stirring occasionally, until tomatoes are thoroughly heated.
In ungreased 13x9-inch (3-quart) glass baking dish, spread half of the fettuccine. Layer with all of the spinach and half of the tomato mixture. Repeat with layers of remaining fettuccine and tomato mixture.
Add nutmeg to cream, and pour over top of casserole; sprinkle evenly with cheese.
Bake uncovered 20 to 25 minutes or until thoroughly heated and cheese is melted.