- Preheat oven to 400 degrees.
- Melt butter until lightly browned in a 12-inch cast iron skillet, and then remove from heat.
- Arrange about ⅓ of the sliced turnips in buttery skillet. Sprinkle with about ⅓ of the thyme, rendered guanciale, kosher salt, and cayenne. Repeat for 2 more layers (a total of three layers).
- Return skillet to the stove over medium heat. Cook gently until bottom layer of turnips is golden brown.
- Add cream, cover skillet with foil, and bake until the turnips are just tender (yield when poked with a paring knife).
- Remove foil and sprinkle evenly with parmesan. Bake, uncovered, until golden and bubbling, 10−15 minutes. Let stand 5 minutes before serving.