Melt the butter in a large saucepan over medium heat. Add the onion and cook for 3 minutes. Add the rice and cook, stirring constantly, for 2 minutes. Reduce heat to medium-low. Add the wine and cook, stirring frequently, until the liquid is absorbed. Add the broth, 1/2 cup at a time, stirring occasionally and waiting until it's absorbed before adding more. It should take about 30 minutes for all the broth to be absorbed. Remove from heat and stir in the Parmesan, salt, pepper, peas and prosciutto. Set aside for a few minutes before serving to warm the peas. Spoon into individual bowls.