Fill a large stockpot with water. Cover and bring to a boil over high heat. Add salt and then stir in the pasta. Cook, stirring occasionally, until al dente according to the package instructions. Drain, reserving 1 cup of pasta water.

While the pasta cooks, heat oil in a large skillet over medium heat. Add onion and cook, stirring frequently, until softened, about 5 minutes. Add prosciutto and cook, stirring frequently, until crispy, about 3 minutes. Add prunes and black pepper, and cook, stirring frequently, until fragrant, about 2 minutes. Add ⅓ cup reserved pasta water and stir until everything is mixed well. Using tongs, transfer the cooked fettuccine to the skillet and toss to coat. Stir frequently until the liquid is almost all absorbed, about 1 minute, and then add an additional ⅓ cup reserved pasta water. Continue to stir frequently until almost all the liquid is absorbed, about 1 minute. Add ¼ cup cheese and mix well. Mix in 2 to 3 additional tablespoons reserved pasta water to help incorporate the cheese and toss well. Remove from the heat.

Mix in the remaining ¼ cup cheese; toss well until the pasta is coated and almost all the liquid is absorbed. Top the pasta with more black pepper, if desired. Serve immediately.