It's a long time since I made it, but I used to make a delicious red lentil soup.
I think it went like this: Chop up an onion and two slices of bacon and fry gently in a tbs butter until soft. Add 8 oz red lentils, a can of chopped tomatoes, a bay leaf and 1 pint chicken stock. Simmer for about half an hour, season with plenty of salt and pepper.
Otherwise, you can check out vegetarian cookery books, like The Bean Book or A Foreign Flavour by Rose Elliot. She uses lots of lentils in her recipes.
Let me know if you try the soup!