Add the red lentils and vegetable stock to a small saucepan, and bring to a gentle boil. Cook for around 20 minutes, until the lentils are thoroughly cooked and beginning to break down. Stir thoroughly, especially towards the end of the cooking time, to ensure the lentils don't stick and to help break them up. If the liquid has all disappeared before the lentils are completely soft, just add a dash more water. Season generously with black pepper, and set aside to cool.