Start by finely dicing the onion.
Slice the leeks (using as much of the green part as possible) and rinse well in a colander to remove soil.
Peel and roughly chop the potatoes and carrots.
Wash and chop the celery.
Heat the butter and oil in a large pan over a medium heat and add all the vegetables and garlic.
Mix well and sweat the vegetables under a lid for about 15 minutes stirring occasionally.
Add the mixed herbs, stock, salt and pepper.
Bring to the boil and simmer for 20 minutes.
Take off the heat, add the milk and blend to a smooth consistency.
Bring back to a gentle simmer, adjust seasoning if necessary and serve with crusty bread.