1.Preheat oven to 180°C. Season shanks and dust with flour.
2.Heat oil in a Large, heavy-based flameproof saucepan on high. Add shanks and cook, turning occasionally, for 3-5 minutes, until evenly browned. Transfer to a plate.
3.Reduce heat to medium. Add pancetta to pan and cook, stirring, for 2 minutes, until starting to brown. Add onions, carrots, thyme and garlic. Cook, stirring, for another 3 minutes.
4.Stir through lentils and cook for 1 minute. Stir through stock and beer, then cover and bring to the boil. Add shanks and bake, covered, for 1-1 1/2 hours, until very tender, turning shanks halfway through cooking.
5.Serve with extra chopped thyme and mashed potato, if desired.