Preheat oven to 350°.
Bring 1 cup broth to a boil in a large nonstick skillet. Add mushrooms, onion, and spinach; reduce heat, and simmer for 10 minutes.
Combine the remaining broth, 1 cup milk, garlic powder, and salt in a medium saucepan; bring to a simmer. Combine remaining milk and cornstarch in a small bowl. Add cornstarch mixture and mozzarella to broth mixture; cook 2 minutes or until thick.
Arrange pasta in bottom of a 3-quart casserole coated with cooking spray; top with spinach mixture and chicken. Spoon broth mixture over chicken, and sprinkle with cheddar. Bake at 350° for 30 minutes or until cheese melts and begins to brown.