Soak the lentils in water to cover for a few hours or overnight. Melt the pancetta fat and cook the onion over moderate heat until tender, about 8 minutes, adding salt and a little cinnamon if desired. (The cinnamon will play up the sweetness of the onion, pancetta and chestnuts.) Add the lentils, bay leaf, thyme and water or stock, and bring up to a boil. Reduce the heat and simmer until the lentils are tender but not falling apart. Add the tomato paste and chestnuts and simmer for 5 minutes. Season to taste with salt and pepper. Garnish with fried bread croutons if desired, and chopped parsley and thyme.