Place the potatoes, tomatoes, onion, garlic, and sausage in a large pot with enough water to cover. Bring to a boil, and cook 15 minutes.
Stir lentils into the pot. Add more water if necessary to just cover all ingredients. Season with salt and pepper. Bring to a boil. Reduce heat to medium-low, and continue cooking 20 minutes, stirring occasionally, until lentils are tender.