Parboil the beans in salted water for 5 minutes, then drain. Cook the pasta shells in boiling, salted water for about 12 minutes until just tender.
Meanwhile, heat the oil, add the prosciutto and fry until crisp; remove with a slotted spoon and keep warm. Fry the garlic for 1 minute. Add the beans and fry gently for 5 minutes, then stir in the tomatoes and cook for 2–3 minutes until starting to soften.
Add the crème fraîche and seasoning and heat through, stirring. Drain the pasta and toss with the sauce, then sprinkle over the prosciutto and Parmesan.