Cook the pasta according to the package directions.
Meanwhile, heat the oil in a large skillet over medium-high heat. Add the onion and 1/4 teaspoon each salt and pepper. Cook, stirring occasionally, until softened, 4 to 6 minutes.
Add the cabbage and broth and cook, stirring occasionally, until the cabbage begins to wilt, 2 to 3 minutes. Stir in the prosciutto, butter, and sage and cook until the butter has melted. Serve over the pasta.
Substitution: Savoy cabbage has thin leaves that cook quickly. If you can't find it in the market, try using Napa cabbage instead. In place of the prosciutto, use deli ham or bacon.