In a medium-sized saucepan over medium heat, mix the lentils and vegetable stock together. Cover and bring to a simmer and cook for 35–40 minutes. If the lentils dry out, top them up with water. You don’t want the lentils to go soft and mushy, but you do want them to cook through, so don’t be shy to taste test while cooking. Remove from the heat and let them cool for 10 minutes.