Rinse the lentils and check over for stones.
Peel and chop the carrots in ½ cm (¼ inch) dice. Chop the celery to the same size. Peel and finely dice the onion.
Heat 1 tbsp oil in a pressure cooker or medium pot. Cook the vegetables in the oil for about 5 minutes over medium heat, stirring occasionally. The goal is to soften the vegetables, not to brown them too much.
Add the lentils, unsalted broth, and water. If using a pressure cooker, cook under pressure for 14 minutes. Make sure that the water level is at a safe level for the size of your pressure cooker. If cooking normally, cook for 30-40 minutes, until tender.
Once cooked, add salt and pepper.