How to Make It
In a large pot, heat the oil over moderate heat. Add the celery, onion, and carrot and cook, stirring occasionally, until the onion is translucent, 5 to 10 minutes.
Add the lentils, tomatoes, water, dried savory, if using, bay leaf, salt, and pepper. Bring to a simmer. Reduce the heat and cook, partially covered, until the lentils are tender, about 40 minutes.
Meanwhile, heat a large nonstick frying pan over moderately high heat. Add the sausage and cook, turning, until browned, about 3 minutes in all. Remove. When the sausage is cool enough to handle, cut it crosswise into 1/4-inch-thick slices.
Stir the sausage and fresh savory, if using, into the soup and simmer it for 5 minutes longer. Remove the bay leaf. If the soup is too thick for your taste, thin it with additional water.