Meatballs with Ricotta: Polpettone con la Ricotta

Name: Meatballs with Ricotta: Polpettone con la Ricotta
Ingredients: 1/2 pound ground pork; 1/4 pound ground veal; 1/2 pound fresh ricotta; 1 egg, beaten; 2 tablespoons Parmigiano-Reggiano, grated; 6 pickled gherkins, chopped; 3 tablespoons chopped pistachios; 2 tablespoons butter; 1 cup brown chicken stock, recipe follows; 1/2 cup milk, optional; Salt and pepper, to taste; 2 tablespoons extra-virgin olive oil; 3 1/2 pounds chicken wings, backs, and bones; 3 carrots, coarsely chopped; 2 onions, coarsely chopped; 4 ribs celery, coarsely chopped; 2 tablespoons tomato paste; 1 tablespoon black peppercorns; 1 bunch parsley stems