Heat the oil in a large heavy pot over medium-high heat. Add the sausages and cook, stirring occasionally, until browned, about 4 minutes. Remove the sausage to a plate. Add the onion, carrots and celery to the drippings in the pot and cook, stirring, until soft and just beginning to brown, about 5 minutes. Sprinkle the mixture with salt and pepper.
Add the garlic, cumin and red pepper flakes and stir until fragrant, about 1 minute. Add the lentils, tomatoes, bay leaf, 8 cups of water and the sausage. Bring to a boil, then reduce the heat to a simmer and cook until the lentils are soft, about 45 minutes. Remove the bay leaf before serving.