Place olive oil, shallots, celery and prosciutto in a 12 to 14-inch deep-sided frying pan and place over medium heat. Cook until soft but not brown, 8 to 10 minutes. Add rice and stir over medium heat for 2 minutes, until rice becomes almost opaque. Add chicken stock to cover rice, turn heat up to high. As level of stock dips below level of rice, continue to add stock, one ladle at a time, to keep rice covered. Stir the rice constantly during this procedure. Taste the rice after 15 minutes: it should still be hard. Add peas and continue cooking, until rice is soft and creamy, yet still al dente. Rice should still be quite wet, but not swimming. Remove from heat and stir in butter and cheese. Adjust for seasoning and serve immediately.