Heat oil in large saucepan or Dutch oven. Add onion, garlic and hot pepper flakes and cook on low heat for 3 to 5 minutes, until fragrant. [I add carrots and celery to the soup at this point, but that's optional] Add cumin and cook for 30 seconds. Stir in lentils and combine well. Add stock, salt and pepper and bring to boil. Reduce heat, cover and simmer until lentils are tender and soup is beginning to thicken, about 25 to 30 minutes. Soup can be pureed, partially pureed or left thick but coarse. Add extra stock or water to thin if necessary. Add lemon juice and taste and adjust seasonings if necessary. Serve sprinkled with cilantro.