Toss macaroni with vegetable oil in a large bowl.
Cook broccoli with 1 teaspoon salt in boiling water to cover 2 minutes or until crisp-tender; drain. Plunge into ice water to stop the cooking process; drain. Add broccoli and spinach to macaroni in bowl.
Melt butter in a saucepan over medium heat; add onion, and sauté until tender. Add mushrooms; cook 5 minutes. Sprinkle with flour, stirring until blended. Gradually stir in milk; cook over medium heat, stirring constantly, 15 minutes. Add 1 1/2 cups cheese, remaining 1/2 teaspoon salt, whipping cream, pepper, and nutmeg, stirring until cheese melts. Stir into macaroni mixture; spoon into a lightly greased 13- x 9-inch baking dish. Sprinkle with remaining 1/2 cup cheese.
Bake at 375° for 40 minutes or until set.