Place an oven rack in the center of the oven. Preheat the oven to 350 degrees F. Butter the bottom and sides of a 9-inch springform pan. Wrap the outside of the pan with foil.
For the rice: Bring the marinara sauce and rice to a boil in a medium saucepan over medium-high heat, stirring occasionally. Reduce the heat to a simmer and cook for 6 minutes. Remove the pan from the heat and set aside to cool.
For the bechamel sauce: Melt the butter in a 1-quart saucepan over medium-low heat. Add the flour and whisk until smooth, 1 to 2 minutes. Gradually add the milk, whisking constantly to prevent lumps. Simmer over medium-low heat, whisking constantly, until the sauce is thick and smooth, about 6 minutes. Remove the pan from the heat and stir in the salt and nutmeg.
For assembly: Mix together the bechamel sauce, rice mixture, 1/2 cup of the Parmesan, prosciutto and peas in a medium bowl. Slowly mix in the eggs, salt and pepper.
Pour the mixture into the prepared springform pan and cover with foil. Place in a shallow roasting pan. Add enough hot water to come halfway up the side of the springform pan. Bake for 1 hour.
Preheat the broiler. Remove the foil and sprinkle the top with the remaining 1/3 cup Parmesan and the breadcrumbs. Broil just enough to melt the cheese and create a crispy crust, about 2 minutes. Cool for 15 minutes. Loosen the risotto from the sides of the pan by running a thin metal spatula around the edge. Remove the sides of the pan. Cut into wedges and serve.