For a really big pot (lots of freezing for future use)-
Bag of lentils
carrots
onions (better than leeks for lentil soup)
Turnip
Celery (necessary ingridient to bring the taste of the lentils out - doesn't taste like lentil soup without it - even if you're not keen on celery - it gets lost in the overall taste!)
Veggie stock
Salt
WHITE pepper (again, lentil soup is better with white pepper rather than just black)
Freshly ground black peper if you like it spicey
I'm know for making a mean pot of soup
and lentil is one of my favourites!
Oh, also better cooked slowly over a long period of time, then left to cool before heating up - seems to be nicer.
Did you get all that?!
DG
I make mine with a smoked bacon joint, a wee bit expensive but worth it Then in go the lentils, carrots, turnip, onions and potatoes - delicious and there is plenty meat to put back in the soup (after liquidising) and also for sandwiches etc. I also add a couple of ham stock cubes as well. It always tastes much better the second day 
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