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.