Melt 2 tbsp butter in a large skillet. Add proscuitto and cook until crispy. In same skillet, add the onion and cook for two minutes (until golden). Pour in the cream and reduce until slightly thickened.
Remove the pasta from the water when it is al dente, about 8 minutes. Drain and add the pasta to the cream mixture along with the peas and cheese. Reduce for 1 minute. Toss and adjust the seasonings. Spoon into a large pasta bowl and serve.
Garnish with additional grated Parmesan and chopped parsley, if desired.