The following recipe is an excerpt from the book The Vegetable Dishes I Can't Live Without by Mollie Katzen.

It's nice to slip in some turnips among the more familiar potatoes and carrots, for something slightly different.

Mollie Katzen's Potato, Turnip & Carrot Gratin with Garlic-Herb Béchamel Sauce

2 tablespoons unsalted butter
1 teaspoon minced garlic 1/2 teaspoon dried thyme 1 1/2 tablespoons unbleached all-purpose flour 1 1/4 cups warmed milk 1 bay leaf Salt, to taste Nonstick cooking spray 1/2 pound unpeeled Yukon gold potatoes 1/2 pound turnips 1/2 pound carrots 1 cup minced shallots Freshly ground black pepper, to taste 1 cup coarse bread crumbs 1/2 cup grated Swiss cheese (Gruyère or Emmenthaler)

Melt the butter in a small saucepan over low heat, adding the garlic and thyme when it is melted.
Whisk in the flour and keep whisking for a minute or so as it forms a thick paste.
Keep whisking as you drizzle in the warmed milk, keeping the mixture moving until there are no lumps.
Add the bay leaf and turn the heat way down. Cook, stirring frequently, for about 5 to 8 minutes, or until smooth and silky. Remove from the heat and remove the bay leaf. Stir in a dash of salt and a few shakes of white pepper, then set aside.
Preheat oven to 375ºF (175ºC). Lightly spray a 2-quart gratin dish with nonstick spray.
Cut the potatoes, turnips, and carrots into very thin slices (about 1/8 inch). For the carrots, do this on the diagonal. Spread the cut vegetables (including the shallots) together in the prepared pan to make a single mixed layer. Sprinkle lightly with salt and black pepper.
Pour the béchamel sauce from step 4 over the top of the vegetables and cover the pan tightly with foil. Bake in the center of the oven for 1 hour, or until the vegetables are fork-tender. Remove the dish from the oven and remove the foil.
Heat the broiler. Sprinkle the bread crumbs and then the grated cheese on top of the vegetables. Broil until the cheese is melted and beginning to form a crust. Serve hot

Makes 4 to 5 servings.

Notes:

It's easiest to warm the milk in the microwave right in its measuring cup.
Good bread crumbs are made by hand from good bread. My favorites for this recipe are either a home-style whole wheat or pumpernickel. Make your own bread crumbs by drying out some of your favorite bread, then crumbling it either by hand (in a plastic bag, so it won't go all over the place) or in a food processor with the steel blade (a few spurts).