Preheat oven to 350-degrees.
Spray 7 x 11-inch baking dish with cooking spray.
In large sauté pan, melt butter and cook leeks for 6 to 8 minutes on medium heat, or until slightly golden brown. Sprinkle flour over leeks and mix well and cook 1 minute. Whisk skim milk in gradually, add thyme and nutmeg then bring to simmer for 8 minutes, constantly stirring.
Assemble lasagna in baking dish starting with three noodles, slightly overlapping each other, add 1/3 of the sauce, half of the grated squash, 1/3 of the cheese, 3 noodles, ½ the remaining sauce, remainder of squash, ½ the remaining cheese, last 3 noodles, remainder of sauce and top with remainder of cheese and toasted pine nuts.
Spray a small piece of parchment paper that will fit the top of the baking dish and lay it sprayed side down on the lasagna, add foil to cover and bake for 50 minutes. Remove parchment and foil and bake 25 minutes, then place under broiler for 2 (2-minute) intervals.
Let cool 20 minutes before serving. Yields: 6 servings.
277 calories; 9 g fat; 4 g sat; 2 g mono; 19 mg cholesterol; 37 g carbohydrates; 14 g protein; 6 g fiber; 164 mg sodium; and 514 mg potassium.