Rinse lentils in a fine mesh strainer and drain. Brown sausage in a skillet drizzled with olive or canola oil. Remove sausage to a paper towel to drain, but leave drippings in the pan.
Sauté lentils in the sausage drippings for a couple of minutes. Remove them to a 6-quart saucepan. Combine lentils with 6 cups of stock and add bay leaf. Bring to a boil for 5 minutes.
Cover and allow reduce to a simmer for 40 minutes. While lentils cook, In the skillet that sausage cooked in, add 1 or 2 teaspoons of olive or canola oil if no drippings remain.
Add carrots, celery, and onion. Salt and pepper. Sauté until they begin to soften. In a small pan, roast fennel, cumin, and coriander seed until fragrant. Grind in a spice or coffee grinder.
Add spices, crushed red pepper, if using, and garlic to sautéed vegetables. Cook for another minute. Using an immersion blender, blend lentils lightly. You still want whole lentils visible.