Put the lentils in a saucepan, cover with water and bring to the boil. Boil rapidly for 10 minutes, then drain well.
Sprinkle the pork with salt and pepper. Heat the oil in a large heavy-based frying pan and brown the meat on all sides. Transfer to a casserole dish and add the lentils.
Add the onions to the pan and fry for 5 minutes. Stir in the garlic, rosemary and stock and bring to the boil. Pour over the meat and lentils and cover with a lid. Place in a preheated oven, 180°C (350°F), Gas Mark 4, for 1 hour.
Stir the carrots into the casserole, season well and return to the oven for a further 20–30 minutes until the pork is cooked through and the lentils are soft. Drain the meat, transfer to a serving platter and leave to rest in a warm place for 20 minutes. Carve into thin slices, then serve with the lentils, carrots and juices.