Heat oil in a 6-quart pressure cooker over medium heat until hot. Add rice and onion, and sauté 1 minute. Stir in wine and broth. Close lid securely, and bring to high pressure over high heat (about 4 minutes). Adjust heat to medium or level needed to maintain high pressure, and cook 3 minutes. Remove from heat, and let stand 5 minutes. Place pressure cooker under cold running water. Remove lid, and stir in the tomatoes, parsley, pepper, and prosciutto. Cook, uncovered, over medium-high heat 3 minutes, stirring constantly. Stir in cheese. Garnish with parsley sprigs, if desired.