For the best flavor, save some cooked beef bones or start with raw soup bones by roasting them in the oven. Remove the marrow if desired (It tends to make soup a bit greasy for my taste, and I save it to spread on toast instead of butter). Cover the bones with water (I think I put in about 16 cups of water), add a small splash of apple cider vinegar or lemon juice, and simmer them in a large pot with a clove or two of garlic (I threw in a whole, uncut peeled clove of garlic). Let these cook as long as you like to give the broth flavor and draw the minerals into the broth from the bones--I let them cook for an hour or two this time, but if you're not highly sensitive to excitotoxins you can let them simmer for a full day or so.
Add a cut-up onion (I used a sweet yellow onion), several cloves of chopped garlic (I used 4) and about 2 to 3 cups of lentils, bring to a boil and let these simmer in the broth while you cut up the rest of the vegetables and add them.
I added sliced baby carrots (probably the equivalent of 2 or 3 full-sized carrots), 3 large stalks of diced celery, 4 sliced green onions, about a half cup of chopped fresh parsely, a small head of chopped broccoli and 3 miniature sweet red bell peppers (probably the equivalent of 1/2 to 1 full sized bell pepper. You could use other vegetables such as cauliflower or cabbage as well.
Once the lentils are tender, remove the bones and add a can of pureed tomatoes (and/or some chopped fresh tomatoes), a splash of wine (optional--I used about 2 tablespoons of cabernet), about a tablespoon or so of dark blackstrap molasses, about a teaspoon of smoked paprika, about a half teaspoon of cumin, two to three teaspoons of sea salt, and a dash of black pepper. Simmer for a few minutes to allow the flavors to blend before serving.
Note: The reason for waiting to add the tomatoes and wine until the lentils are cooked is that adding too much acid while cooking can prevent legumes from becoming tender.