Heat oven to 350°F. Spray 13x9-inch (3-quart) glass baking dish with cooking spray.
Peel, halve lengthwise and seed squash; cut crosswise into 3/4-inch slices; peel parsnips, halve lengthwise and cut crosswise into 1/8-inch slices. Layer the squash slices in bottom of pan; top with the parsnips.
In heavy 2-quart saucepan, melt 1/4 butter over medium heat. Stir in flour, salt, nutmeg and pepper. Cook and stir 1 minute; remove from heat. Gradually stir in milk. Cook over medium heat 5 to 7 minutes or until thickened and bubbly. Remove from heat; stir in lemon juice and cheese. Pour cheese sauce over squash mixture. Cover with foil.
Bake 40 minutes. Meanwhile, mix bread crumbs and 3 tablespoons melted butter. Remove foil. Use spoon to press down on vegetables to moisten evenly; sprinkle with crumb mixture. Bake uncovered 25 to 30 minutes longer or until vegetables are tender. Let stand 5 minutes before serving.