Trim the tough bottoms of the asparagus, peel the lower green portion and cook in salted boiling water until tender, about 6 minutes. Reserve ½cup of the water the asparagus cooked in. Cut the asparagus into pieces about ½-inch long.
Fill a pot for the pasta with about 6quarts of water, cover, and place over high heat
In a 10-inch skillet heat the butter over medium high heat until melted and add the onions. Sauté until the onions have turned a rich caramel color, about 10 minutes. While they Sauté, cut the prosciutto.
Add the prosciutto and cook until it has lost its raw color, about a minute.
Add the asparagus, raise the heat to medium high and cook until lightly colored, 2 to 3 minutes. Add the asparagus water. When the water has evaporated add the cream and some black pepper. Continue cooking until the cream has reduced by half. Remove from the heat and set aside.
Add about 2tablespoons salt to the boiling pasta water, put in the fettuccine, and stir until all the strands are submerged. Cook until al dente. When the pasta is done, drain well, toss with the sauce and the Parmigiano, and serve at once