Heat 1 1/2 teaspoons butter in a skillet over medium heat. Add leek and zucchini; sauté 6 minutes or until lightly browned.
Combine milk and next 5 ingredients (milk through eggs) in a bowl; stir with a whisk. Stir in leek mixture and spaghetti.
Heat 1 1/2 teaspoons butter in skillet over low heat. Add egg mixture. Cover; cook 10 minutes or until almost set. Top with cheese. Wrap handle of skillet with foil. Broil 5 minutes or until cheese melts.