Heat the butter and oil in a frying pan and gently sauté the onions until they soften. Add the chicken and prosciutto and fry with the onions until they are nearly cooked through (the chicken should still be a little pink). Set aside and leave to cool.
Transfer the mixture to a food processor and pulse until a smooth paste. Add the egg, cheese and parsley, and pulse until well combined. If the mixture is too wet, you can thicken it with a few spoonfuls of fresh breadcrumbs. If it’s too dry, slacken with a few spoons of cold chicken stock. Set aside.
To make the Tortellini, roll the pasta through a pasta machine, starting at setting number 1, then setting 5 and finally the last setting (8 or 9). Make sure you set the machine up on a lightly floured work surface to prevent the pasta sticking. Cut the pasta into 6 cm discs. You can use a special pastry cutter, or improvise.
Place spoonfuls of the filling onto each pasta disc and lightly brush the edges with egg wash. Fold the pasta shapes into halfmoons, sealing the edges well.
To create the traditional tortellini shape, fold the edges together around your index finger and stick them together. Bring the stock to the boil and drop in the tortellini. Lower the heat and simmer for about 4 minutes or until the tortellini are cooked through. Serve with freshly grated Parmigiano Reggiano.
Bring the stock to the boil and drop in the tortellini. Lower the heat and simmer for about 4 minutes or until the tortellini are cooked through. Serve with freshly grated Parmigiano Reggiano.