Rinse lentils and remove any small rocks, if necessary.
Add lentils to a large pot, then fill with either chicken stock, water, or a mixture of the two until the liquid comes to about 1 inch above the lentils. I find that water and Better Then Bouillon is an easy way to accomplish this.Note: By the end of the cooking process, I think I had added a total of 72oz (or 9 cups, or 2.12L) of liquid to the pot. Add as you go though; lentils might have different absorption rates, especially if you've soaked them beforehand. In a skillet, heat some oil and add diced onion, chopped tomatoes, and garlic. Add the oregano and optional fennel now as well, then cook until onions are softened. Add to pot.
Add Italian sausage to skillet and cook until browned. Add to pot.
Once all ingredients have been added to the pot, bring to a boil and reduce to a simmer.
Simmer for 30 to 45 minutes, until the lentils are soft.
Add salt and freshly ground pepper to taste, then stir it all into the soup.
Serve, perhaps with some crusty bread on the side.
