They should be sliced thin (1/8 inch thick). You can use a food processor, a mandolin or a good old fashioned knife. I love this mandolin, just set it to the thickness you want and you can bang those puppies out in no time.
Turnip Gratin with Almonds (Food Network Magazine)
2 lbs turnips (about 5 medium)
1 cup plus 2 Tbs heavy cream
1 Tbs all-purpose flour
2 tsp fresh thyme
1 clove garlic, finely grated
1/4 tsp lemon zest
1/4 tsp fresh grated nutmeg
Large pinch of ground cinnamon
salt and pepper
2 Tbs unsalted butter
1 cup fresh breadcrumbs (or panko)
3 Tbs sliced almonds, roughly chopped
2 Tbs chopped fresh parsley
Preheat the oven to 350. Peel and slice turnips 1/8 inch thick. Whisk cream, flour, 1 tsp thyme, garlic, lemon zest, nutmeg, cinnamon, 1 tsp salt and pepper to taste in a large skillet over medium heat. Add the turnips and bring to a simmer over medium heat; cover and cook until softened, about 3 min.
Stir the turnip-cream mixture and spread evenly in a 2 quart baking dish. Cover with foil until turnips are very tender, 35-40 min. Uncover and continue baking until cream mixture is slightly thickened and top is golden, about 10 more min.
Meanwhile, make the topping: Melt the butter in a large skillet over medium heat. Add the breadcrumbs, almonds and remaining 1 tsp thyme and cook, stirring, until the breadcrumbs are toasted about 4 min. Stir in the parsley and a pinch of salt. Remove gratin from the oven and sprinkle with breadcrumb topping.