Heat olive oil in a large pot over medium heat. Add the celery, leek, carrots, fennel, garlic, salt and crushed red pepper and sauté until softened and the onions begin to caramelize. Add the potatoes, lentils, stock and water. Bring the soup to a simmer.
Lower the heat, cover the pot, and cook for 45-60 minutes until the lentils are tender and the potatoes are cooked through.
Serve piping hot with a drizzle of basil oil on top.