Roman lentil soup

This is a recipe from Nourishing Traditions. We all really liked it - I can see it becoming a regular fixture.

1 cup red lentils (soaked for 7 hours)
2 leeks or onions, chopped
2 carrots, peeled and chopped
1 tbsp olive oil
1 tbsp butter
1 litre stock (beef or chicken - or veg if you are vegetarian)
1 tsp fresh thyme
1/2 tsp crushed green peppercorns
Fish sauce or sea salt to taste
Splash of lemon juice or whey
Creme fraiche to serve (optional)

Gently fry the leeks and carrots in the butter and olive oil for half an hour. Add the stock and lentils and bring to the boil. Skim off the foam that rises and then reduce heat. Add the thyme and peppercorns and simmer for another half hour. Liquidize and stir through the lemon juice or whey. Season to taste and serve with creme fraiche.