Special equipment: an immersion blender
In a large bowl, combine the bread with 1/2 cup of the milk and break apart with your fingers. Mix well and allow the bread to soak for 5 minutes. Add the shallots, garlic, egg, prosciutto, Parmesan and 1/4 teaspoon salt and mix with a wooden spoon to evenly distribute the prosciutto. Add the turkey and parsley and mix well. Divide the mixture into 2 even oblong loaves.
Heat 2 medium skillets over medium-high heat. Add 1 tablespoon olive oil and 1 tablespoon butter to each pan. Add a loaf to each pan and cook on all sides until golden brown, 3 to 4 minutes per side. Divide the white wine between the two pans and cook until reduced by half. Add the remaining 2 cups milk and 1/4 teaspoon salt to one of the pans and bring to a simmer. Add the marinara to the other pan and bring to a simmer. Reduce the heat under each pan to medium low and cover the pans. Braise until an instant-read thermometer registers 160 degrees in the center of each loaf, about 25 minutes, carefully flipping the loaves about halfway through.
Remove the loaves from the pan and allow them to rest 10 minutes before slicing. Remove the marinara from the heat and stir with a wooden spoon. Simmer the remaining milk, uncovered, until it reduces by half. It will be slightly brown and look broken. Using an immersion blender, blend the milk until fully emulsified and creamy. Serve each polpettone with its respective sauce.