1/2 cup grated Parmigiano-Reggiano, plus more for sprinkling
Freshly ground black pepper
1 pound fettuccine or spagetti
Heat the oil in a 12-inch skillet over medium-high heat. Add the ground lamb and brown slowly, scraping the bottom of the pan and reducing the heat as necessary to keep the lamb from browning too fast, until the moisture evaporates and the meat is almost fried, 15 to 20 minutes.
Add a pinch of salt and the red pepper flakes and cook for 1 minute more. Drain off the fat.
Turn the heat to high, add the thyme, bay leaves, and wine, and stir well. Bring to a boil and reduce until only a small amount of liquid is left in the pan, 5 to 10 minutes.
Add the marinara and the cheese rind, if using, and simmer for 20 minutes, or until the sauce thickens.
Meanwhile, bring 4 quarts water to a boil in a large pot, add 2 tablespoons salt.
When the sauce has thickened, remove and discard the thyme sprigs, bay leaves, and cheese rind.
Stir in the butter, cream, and grated cheese, bring to a simmer, and remove from the heat. Season to taste with salt and pepper.
Cook pasta, until al dente. Drain.
Combine the pasta and sauce, stir. Add remaining cheese and serve.