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 and set aside.
In a small bowl, whisk the eggs, millk, and salt. Stir in the spinach, cheese, bread crumbs, and onion.
Place half of the parsnip potatoes at one end of a 13 x 9 inch baking dish coated with cooking spray. Add squash to the dish forming a stripe next to the parsnip potatoes. Repeat with spinach. Place remaining parsnip otatoes at the opposite end of the dish. (Dish will be full).
Cover and bake at 350 for 45-55 minutes or until a meat thermometer inserted in the middle reads 160 degrees.