In a large stockpot, bring salted water to a rapid boil.
In a large saute pan over medium heat, add the olive oil and butter. When the butter is melted, add the onions and saute until translucent, but not browned, about 1 minute. Add the prosciutto and cook 1 more minute, stirring. Add the cream and bring to a simmer. Simmer until the sauce is just thick enough to lightly coat the back of a spoon. Season, to taste, with salt and black pepper. Add the peas and remove from the heat.
Cook the pasta to al dente. Return the sauce to the heat. Drain the pasta and add it to the sauce. Toss well and season with salt and pepper. Finish with parmesan and more cracked black pepper.