Bring 4 to 5 quarts of salted water to a boil and add the fettuccine; stir periodically.
While fettuccine cooks, melt the butter in a sauté pan over medium heat until just foaming. Gently lay the sage in the pan and heat until the leaves crisp up, about a minute.
Ladle in 1 cup boiling pasta water; stir the sauce, and simmer for about 2 minutes, reducing liquid by half. Grind the black pepper directly into the sauce.
Keep the sauce hot over very low heat; return to a simmer, and immediately add cooked pasta to the sauce. Toss with the sauce, then remove from heat, and sprinkle with cheese just before serving.