Wash lentil and place it into the slow cooker. Meanwhile, heat oil in the pan and saute' garlic and onion. Add in the sliced sausage and saute' until cook through or when it is not pink. Turn the heat off and drain excess oil if desired. Add the cooked sausage, garlic and onion mixture into slow cooker. Add in 4 cups of water and turn the slow cooker in low setting and let the lentil cook for 3 to 4 hours. When the lentil is ready, add salt to taste and add more water if necessary. Cook for another 5 minutes. Serve hot. You can also let the soup cool and place into a ziploc and freeze.