Begin by preparing the vegetables. Start with an onion. Remove the outermost layer, then wash and chop it coarsely.
Peel and wash the carrots, and then coarsely chop them too.
Now, you can use a food processor to chop the onion and carrots finely. If you don't have one, you can do this manually with a knife.
Put the prepared onion and carrots into a large pot along with water, stock cubes, and lentils.
Bring the mixture to a boil, then lower the heat and let it simmer for 30 minutes. During this time, the lentils will soften and begin to dissolve into the soup, while the onion and carrot add flavor and texture.