Trim and remove stems from mushrooms. Quarter large mushrooms. Melt 1 tablespoon of butter or margarine in a large skillet over moderate heat. Stir-fry mushrooms caps and stems 1 minute until coated. Remove. Add 1 tablespoon oil, broccoli and carrots to the skillet. Stir-fry 1 minute. Add 2 tablespoons of water; cover and cook 3 minutes until vegetables are crisp and tender; drain. Melt 4 tablespoons butter or margarine; stir in 3 tablespoons flour. Cook 2 minutes, stirring once or twice. Continue stirring and slowly add light cream. Increase heat slightly; stir in 1 cup Parmesan and 1 cup sharp Cheddar cheese. Add half teaspoon salt and quarter teaspoon pepper. Stir 2-3 minutes until sauce thickens and bubbles. Gently mix ziti which has been cooked and drained, mushrooms, carrots, broccoli and 1 cup walnuts in large bowl. Add cheese sauce and toss gently. Place into a 2 litre baking dish. Cover tightly with foil and refrigerate. Bake at 190ºC or 25 minutes. GOOD DISH TO MAKE AHEAD AND SERVE LATER.