In large sauce pot, cook pasta 2 minutes less than label directs. Drain and return to pot.
Meanwhile, preheat oven to 375°F. In microwave safe 4 cup liquid measuring cup or medium bowl, heat water in microwave oven on high 11⁄2 to 2 minutes or until boiling. Stir in porcini mushrooms; let stand 15 minutes. With slotted spoon, remove porcini; rinse to remove any grit. Finely chop and set aside. Strain soaking liquid through sieve lined with paper towel and set aside.
In 4 quart saucepan, melt margarine over medium heat. Add onion and cook 8 to 9 minutes or until tender and lightly browned, stirring occasionally. Stir in porcini and thyme.
Sprinkle flour over onion mixture in saucepan; cook 1 minute, stirring. Whisk in milk, mushroom liquid, nutmeg, salt, and pepper. Heat to boiling over medium high heat; cook 2 to 3 minutes or until mixture thickens slightly, stirring frequently.
Add porcini sauce to pasta in sauce pot. Stir in Fontina, Ricotta, and 1⁄2 cup Parmesan until combined. Spoon pasta into 3 quart ceramic baking dish; sprinkle with remaining Parmesan. Bake 30 minutes or until center is hot and top is golden.