Place potatoes and parsnips in a large saucepan; cover with water and bring to a boil. Reduce heat; cover and simmer for 15-20 minutes or until tender. Drain. Mash with sour cream, milk and salt; set aside.
Place squash in another large saucepan and cover with water. Bring to a boil. Reduce heat; cover and simmer for 15-20 minutes or until tender. Drain. Mash with salt and nutmeg; set aside.
In a small bowl, whisk the eggs, milk and salt. Stir in the spinach, cheese, bread crumbs and onion.
Place half of parsnip potatoes at one end of a 13-in. x 9-in. baking dish coated with cooking spray. Add squash to dish, forming a stripe. Repeat with spinach. Place remaining parsnip potatoes at opposite end of dish. (Dish will be full.)
Cover and bake at 350° for 45-55 minutes or until a meat thermometer inserted in the spinach reads 160°. Yield: 14 servings (1 cup each).