Heat the olive oil in a large soup pot or Dutch oven over medium heat. Add the onion, carrots, and celery; saute for 3 to 5 minutes, or until the vegetables begin to soften. Add the sausage and brown, breaking it up into bite-size pieces as it cooks. Then add the garlic, and saute another minute.

Into the soup pot, add the water, broth, dry lentils, thyme, cumin, and tomato paste; season with salt and pepper as needed. Bring the soup to a boil; reduce the heat to a simmer and cover. Cook the soup for 40 to 45 minutes, or until the lentils are soft. Taste to adjust the seasoning and add the spinach leaves. Stir until the spinach has wilted, 1 to 2 minutes. Ladle into bowls for serving. Garnish with Parmesan or mozzarella cheese and red pepper flakes to spice it up if desired.