Flatten chicken slightly; sprinkle with garlic salt and pepper. In a large nonstick skillet, cook chicken in butter and oil over medium heat until no longer pink, 3-4 minutes on each side. Remove and keep warm.
In the same skillet, saute the mushrooms, onion, red pepper and pine nuts until vegetables are tender. Add spinach; cook and stir until wilted, 2-3 minutes. Stir in flour. Gradually stir in broth and mustard. Bring to a boil. Reduce heat; cook and stir until thickened, 2 minutes.
Return chicken to the pan; heat through. Sprinkle with cheese. If desired, served with pasta and top with basil.
