Heat oil in heavy 2-quart or larger saucepan or pot over medium–high heat. Add onions, celery, carrots, and garlic; sauté until vegetables soften and begin to brown, about 10 minutes. Add the stock or broth, lentils, tomatoes with juice, herbs and bacon or ham (if using). Be sure there's at least an inch of water above the lentils. Bring to boil. Reduce heat to medium–low, cover, and simmer until lentils are tender, adding water or broth as needed. This should take about 30 minutes, or as specified on the package.