Cook pasta according to package directions. Heat oven to 375°. In large skillet over medium heat, heat 1 tablespoon oil; add green pepper. Cook until tender, turning occasionally, about 10 minutes; remove from skillet. Sprinkle chicken with garlic salt. Heat remaining oil in skillet; add chicken. Cook until no longer pink, about 5 minutes, stirring occasionally.
In 13 x 9 x 2-inch baking dish, spread 1/2 cup spaghetti sauce. Place 3 pieces pasta lengthwise over sauce. Spread about 2/3 cup sauce over pasta. Layer one-third EACH green pepper rings, chicken and mushrooms over sauce. Sprinkle with 1 cup mozzarella cheese. Repeat layer, beginning with pasta. Top with remaining 3 pasta pieces. Spread remaining sauce on top; cover with foil.
Bake 30 minutes; remove foil. Sprinkle with remaining cheese; bake 10 minutes or until hot and bubbly. Let stand 10 minutes before cutting.