Heat the oil in a pan over a medium heat and add the onion and garlic. Cook for 3 minutes, stirring continuously, until soft but not coloured, then add the lentils and taco seasoning and cook for 1 minute more.
Pour the stock into the pan and bring to the boil. reduce the heat to a simmer and leave to cook for 15 minutes until the liquid has dissolved and the lentils are tender.
Place the tortillas on a plate and spoon over the lentil mixture, add toppings of your choice, then fold the tortillas to form a taco.