How to Make It
In a large pot of boiling salted water cook the fettuccine until al dente. Meanwhile, in a medium skillet, melt the butter over moderate heat until it begins to foam, about 1 minute. Reduce the heat to low, add the rosemary and cook until fragrant, about 2 minutes. Season the sauce with salt and pepper, remove from the heat and keep warm.
Drain the pasta, reserving 1/3 cup of the pasta cooking water. Add the pasta and the reserved water to the skillet and toss to coat with the sauce. Transfer to warmed bowls and serve immediately. Pass the Pecorino at the table.