Finely chop the onion and prosciutto or pancetta together into a mince. In a large braiser or sauté pan, sauté this mince gently sautéed in olive oil (or, if you want to be truly authentic, lard).
Lightly dust the cubes of lamb meat with flour and add to the pan, turning up the heat a bit. Allow the meat to brown lightly—taking care not to burn the onion—and season with salt and pepper. Add a splash of the dry white wine and allow it to evaporate completely.
Add enough water to almost cover the meat, lower the flame and cover. Let the lamb braise until tender, normally 60-90 minutes.
Shortly before the meat is done, beat two egg yolks in a small bowl together with the juice of a freshly squeezed lemon and finely chopped parsley.
When the meat is fork tender, remove from the heat. Add a spoonful of the cooking liquid to the egg and lemon mixture to temper it. Then pour the mixture immediately over the lamb and stir until well incorporated.
Keep stirring gently, until the egg has thickened the cooking liquid into a smooth, silky consistency that coats the meat nicely.
Serve immediately.