Step 1Combine the stock and water in a large pot and bring to a boil. Reduce the heat to maintain a faint simmer.
Step 2Melt 2 tablespoons butter in a large skillet or wide-bottomed pot over medium heat. Add the prosciutto and cook until it renders some of its fat, about 3 minutes. Add the onions and cook until tender and translucent, another 3 minutes. Add the rice and cook, stirring constantly, until the kernels are coated with the fat and you can see a translucent area around the outside of each one, surrounding a solid "eye" in the center.
Step 3Begin adding the stock, one-half cup to three-fourths cup at a time. The skillet should be hot enough that there is a loud sizzle each time the liquid is splashed in. Cook, stirring, until the liquid is almost absorbed by the rice; there should be a thin layer of liquid in the bottom of the pan. Repeat until the rice is tender but not mushy. It should be neither chalky in the center nor pasty. It should take about 17 minutes in all and use up most, if not all, of the stock.
Step 4With the last addition of stock, taste and season with salt if necessary. Stir in the sugar snap peas. Cook until they are bright green, about 3 to 4 minutes, and the rice is coated in creamy broth. Remove from the heat and vigorously stir in the chives, Parmesan and the remaining 2 tablespoons of butter. Serve immediately.