Cut off the ends and green parts of leeks. Wash them carefully in order to remove any soil, cut them in pieces of 3 cm (1 inch) length.
In a large pan heat the olive oil, sautee the onion and the leeks for approx 10 minutes.
Add gradually into the saucepan the tomatoes, potatoes, carrots, parsley and the celery.
Season with salt and pepper.
Add 1 cup of water and simmer until tender (20 - 30 minutes) and all the water is absorbed (add more water if required) Remove from heat and stir in the lemon juice, one tablespoon first, taste and add more if you like.