Why turnips and not potatoes?
You could make this with potatoes, but turnips have much more flavor. And turnips have fewer calories, too.
How to make turnips gratin:
Preheat the oven to 350 F. Slice two large turnips very thin (about 1/8 inch thick). In a saucepan over medium heat, melt 1/4 stick of butter and add two tablespoons of flour. Mix and cook for about half a minute, stirring. Slowly add a cup of whole milk, stirring, until the milk thickens and becomes a white sauce. Add 1 cup of grated Gruyere cheese, and stir until it melts. Add salt and pepper to taste (but be careful: there's already salt in the turnips and the cheese).
Grease a 9x9 inch baking pan or casserole dish with butter or olive oil, and put a layer of turnips in the bottom. Pour some of the sauce over the turnips, and keep layering turnips and sauce until the pan is full. End with a layer of sauce. Bake for an hour or until soft and brown on top. Serve with a green salad, or with sauteed broccoli or spinach.
Vegetarian or vegan?
This is vegetarian, but it's not vegan.
Can I keep leftover turnips gratin for later?
Yes, pizza's good cold or reheated for a couple of days as leftovers. Leftover pizza makes an excellent lunch to take to school or work.