Preheat oven to 375°F. Simmer whipping cream, chicken broth, garlic and sage in heavy medium saucepan until slightly thickened, about 5 minutes. Add blue cheese and stir until melted. Remove from heat.
Combine potatoes and turnips in large bowl. Season with salt and pepper. Arrange half of potato mixture in 13x9x2-inch glass baking dish. Pour half of cream mixture over. Repeat layering with remaining potato mixture and cream mixture. Bake until vegetables are tender, about 1 hour 15 minutes. Let stand 15 minutes before serving.
