Combine first 6 ingredients in a large bowl; stir in squash.
Melt margarine in a large nonstick skillet coated with cooking spray over medium- high heat. Add potatoes and leek; cook, stirring constantly, 7 minutes or until lightly browned. Add egg mixture to skillet. Cover, reduce heat to low; cook 10 minutes or until center is almost set. Uncover and sprinkle with cheese.
Preheat broiler. Wrap handle of skillet with foil; broil 5 minutes or until cheese melts.