You want the slices of turnip to be as thin as possible so they cook faster. If you have a mandoline, that would work great. I just use a sharp knife. To make slicing easier, cut the turnips in half so they lay flat, and then cut the slices into half-moons.
2 tablespoons unsalted butter
2-1/2 pounds turnips, trimmed, sliced as thinly as possible
1 tablespoon chopped fresh thyme (or 1 teaspoon dried)
Kosher salt and cayenne pepper to taste
1 cup heavy cream
1/2 cup grated Parmigiano-Reggiano or Romano cheese
Melt the butter in a 12-inch ovenproof skillet over medium heat. Carefully place one third of the turnip slices on top of the butter, overlapping to cover the whole bottom of the pan. Sprinkle with one third of the chopped thyme, some salt, and a tiny bit of cayenne. Repeat with two more layers. Continue to cook uncovered until the underside is browned, about 5 more minutes. Add the cream, cover, and cook over low heat until the center is tender, 15 to 20 minutes.
Uncover, sprinkle the top evenly with cheese, and set the pan under the broiler until the cheese melts and browns in spots. Times will vary. Check often to avoid burning. Let stand 5 minutes before serving.
Source: Adapted from Gourmet (specifically, Holly Smith of Cafe Juanita outside Seattle)