Heat the oil in a wide, medium-sized saucepan, or the base of the Flameware small tagine, and fry the cubed pancetta until beginning to crisp.
Add the lamb, breaking it up with a fork in the bacony pan as it browns.
Tip in the caramelized onions, Marsala, tomatoes, lentils and water and bring the pan to the boil.
Cover with the tagine lid, if using, and simmer the ragu for 20 minutes stirring occasionally. Sprinkle with the cheese (if using) before serving.