Place turnips in a large saucepan, and cover with water. Bring to a boil. Reduce heat, and simmer for 5 minutes. Add potatoes to pan, and simmer for 8 minutes or until potatoes are almost tender. Drain. Sprinkle turnips and potatoes evenly with 1/4 teaspoon salt, and arrange slices in an 11 x 7–inch baking dish coated with cooking spray.
Combine 1/4 cup milk, flour, and 1/4 teaspoon thyme in a medium saucepan, and stir with a whisk until well blended. Gradually add remaining 3/4 cup milk, stirring with a whisk until smooth. Stir in remaining 1/4 teaspoon salt and 1/4 teaspoon black pepper. Cook over medium heat for 7 minutes or until thick, stirring frequently. Remove from heat, and add Gruyère cheese, stirring until smooth. Pour cheese mixture over turnip and potato slices, and toss gently.
Combine seasoned breadcrumbs and butter; sprinkle over turnip mixture. Bake at 425° for 15 minutes or until thoroughly heated. Serve immediately.