Butter the inside of a 10-inch nonstick ring mold very well all the way to the top. Coat the inside with the breadcrumbs, tapping out the excess, and set aside.
Add the chicken broth to a medium saucepan along with the salt and 1 cup water. Bring to a boil over high heat. Add the rice and cook, stirring often, until the liquid is absorbed, the rice is just under al dente and the mixture is creamy, about 10 minutes.
Place the mozzarella, Taleggio, Parmesan and garlic in a large bowl. Add the hot rice to the cheese mixture. Using a rubber spatula, stir well to combine until the cheeses are melted. Add the cream to moisten. Fold in the black pepper, scallions and lemon juice. Spoon the rice mixture into the prepared ring mold and press down to pack it in tightly. Place on a rimmed baking sheet and bake until lightly browned on top and bubbly, about 25 minutes.
For the sauce: Meanwhile, combine the tomato puree, mascarpone, salt, basil and Parmesan rind in a medium skillet and bring to a simmer over medium heat. Simmer, stirring often, for about 15 minutes. Keep warm to serve.
For the peas: Heat a medium skillet over medium-high heat. Add the oil and shallots to the skillet and cook until fragrant and beginning to soften, about 3 minutes. Add the peas and salt and cook until bright green and warmed through, another 5 minutes. Keep warm.
To finish: Let the rice mold cool slightly for 10 to 15 minutes. Place a large, flat plate over the top of the mold and, using a kitchen towel as hot pads, carefully invert the rice onto the plate. Cover the entire rice ring with the slices of prosciutto cotto, leaving the hole open in the center. Spoon some of the peas into the center and decorate with Italian parsley sprigs, if using. To serve, use a serrated knife to cut slices. Serve with the peas and sauce.