Preheat oven to 400°.
Place parsnip in a saucepan, and cover with water. Bring to a boil, reduce heat, and simmer over medium heat 20 minutes or until tender. Drain. Place the parsnip in a bowl, and mash with a potato masher until smooth.
Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add onion and garlic; sauté 3 minutes or until onion is tender. Add flour, stirring to coat onions. Gradually add 1/2 cup milk, stirring constantly with a whisk until blended. Add the remaining milk; cook over medium heat until thick (about 8 minutes), stirring constantly. Remove from heat, and stir in salt, pepper, and nutmeg. Add milk mixture and sage to mashed parsnip, and stir to combine. Place the parsnip mixture in a shallow 1 1/2-quart baking dish coated with cooking spray.
Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1/2 cup. Combine breadcrumbs and butter, and sprinkle evenly over parsnip mixture. Bake at 400° for 25 minutes or until golden brown.