Preheat oven to 375 degrees F. Bring a large pot of water to a boil. Cook spaghetti 2 minutes less than package time, until almost done but still firm in the center. Drain well and rinse under hot water; drain again.

Meanwhile, in large skillet heat butter over medium heat. Cook onion, garlic, and mushrooms until tender, about 4-6 minutes. Add chicken; cook and stir for 2 minutes. Add Alfredo sauce, milk, and mustard and stir; bring to a simmer. Stir in drained pasta, tomatoes, Swiss cheese, and half of the Parmesan cheese; stir gently until combined.

Pour mixture into 3 quart casserole dish. Top with remaining Parmesan cheese. Bake the Chicken Tetrazzini, uncovered, for 40-50 minutes or until casserole is bubbling, cheese has melted and is golden brown around edges.