Peel, quarter, and thinly slice rutabaga. Peel and thinly slice potatoes; set aside.
Melt butter in a heavy saucepan over low heat, and add flour, stirring until smooth. Cook, stirring constantly, 1 minute. Gradually add whipping cream, and cook over medium heat, stirring constantly, until mixture is thickened and bubbly. Stir in salt and next 3 ingredients. Set sauce aside.
Sprinkle Parmesan cheese in a 10-inch cast-iron skillet lined with parchment paper or aluminum foil, and arrange half of rutabaga slices over cheese. Layer with 3/4 cup sauce, potato slices, and 3/4 cup sauce. Arrange remaining rutabaga slices on top, and add remaining sauce. Cover skillet tightly with aluminum foil.
Bake at 400° for 1 hour; uncover and bake 20 more minutes. Let stand 15 minutes. Invert onto a serving platter, and garnish, if desired. To serve, cut into wedges.