To make the stock, heat the oil in a large saucepan, add the vegetables and fry for 2–3 minutes, then add the vegetable trimmings and bouquet garni and season well. Pour over the water, bring to the boil and simmer gently for 1½ hours, by which time the stock should have reduced to 900 ml (1½ pints). Drain over a bowl, discarding the vegetables and retaining the stock.
Heat the oil in a medium saucepan, add the leek and garlic and fry over a gentle heat for 5–6 minutes until the leek is softened.
Add the lentils, stock and herbs, bring to the boil and simmer for 10 minutes. Add the peas and continue to cook for 5 minutes.
Transfer half the soup to a liquidizer or food processor and blend until smooth. Return to the pan, stir to combine with the unblended soup, then heat through and season with plenty of pepper.
Stir together the crème fraîche and mint and serve on top of each bowl of soup.