Bring a large pot of lightly salted water to a boil. Cook pasta 9 to 11 minutes, or until just tender.
While pasta is cooking, heat oil in large nonstick skillet over high heat. Add onion and broccoli, and cook, stirring, 2 minutes. Add mushrooms and 1/2 teaspoon garlic salt, and cook 3 to 4 minutes, or until lightly browned. Transfer vegetables to bowl and set aside.
In same skillet, melt butter over medium heat. With wire whisk, stir in flour and cook 2 minutes, or until lightly browned, stirring frequently. Whisk in about 1/2 cup milk to form a smooth paste. Gradually whisk in remaining milk and cook, stirring constantly, until sauce boils and thickens. Reduce heat to low and stir in cheese, pepper, nutmeg, and remaining 1/2 teaspoon garlic salt. Simmer, stirring occasionally, 2 minutes.
Drain pasta and return to pot. Toss with sauce and vegetables. Serve immediately.