Dump the lentils in your largest pan and add a LOT of water (at least 750 ml to start with).
Crumble in the stock cubes, and add the onion, preferably grated (if not, then very very finely chopped). If you like garlic, add as much crushed garlic as you feel like.
Cover and start cooking over a high heat.
Now for the technique (blender-free): when the water starts boiling, the lentils will start fluffing up and turning yellow. You have two tasks: to stir briskly and frequently (to prevent sticking AND to break up the lentils), and to add about a cup of water every time the lentils seem to have absorbed most of the liquid (if it's boiling water from the kettle, you do save time). An MP3 player is a useful accessory here while you stand by the pot.
After about 30 minutes, the lentils should have gone from yellow to beige, and have lost their lentil-like form - they should have pretty much broken up and dissolved. This is NOT a recipe for smooth, liquid soup involving hours of painstaking rubbing through a sieve - at this point, you CAN use a blender or even electric hand whisk for a smoother soup - but I much prefer this more rustic version where you can actually feel the texture of the lentils.
Stirring all the time, add enough water for the soup to reach the consistency you prefer - it's supposed to be thick, so it's better not to go over the top!
Now you can add salt to taste. My downstair neighbours liked to add a hefty dose of red chilli flakes, but I always feel that the simpler you keep this, the better.
Just before serving, or after you've ladled the soup into bowls, you can add a drizzle of olive oil.
Serve with plenty of bread and a plate of lemon wedges - lemon is absolutely perfect squeezed over this soup.
You now have a large potful of extremely cheap, nourishing and tasty soup to heat up for days to come!