Preheat oven to 350°.
Heat oil in heavy, sauté pan with lid over medium-high heat. When hot, add pork and sauté for about 6 minutes, turning to brown all sides. Remove pork and drain on a paper towel.
Add onions, carrots, potatoes and mushrooms to the pan and sauté for about 5 minutes or until just softened. Stir in drained pork, lentils, Swiss chard, tomato paste, stock, salt and pepper. Cover and place in preheated oven. Bake for about 45 minutes or until liquid has been absorbed and meat is cooked. If liquid is absorbed before meat is done, pour in small amounts of hot stock or water as needed. About 5 minutes before done, remove lid and sprinkle top with grated cheese, if desired. Bake, uncovered, for remaining 5 minutes. Serve hot.
Variations: You may substitute any meat or game for the pork. If using game or duck, use half stock and half red wine as the liquid.