Heat the oil in a large saucepan
and add the onion, leek, celery, garlic and bacon. Cook, stirring, over medium heat
for 4-5 minutes, or until the vegetables are softened. Add all the stock, bring to the
boil, then reduce the heat and simmer for 25 minutes. Stir in the frozen peas and mint,
return to the boil over high heat, then reduce the heat and simmer
for a further 15 minutes. Leave the soup to cool slightly
(for safety reasons) before transferring to a blender and blending
until smooth. Season to taste with salt and
cracked black pepper, then stir in the lemon juice. Refrigerate for at least 3 hours,
or until well chilled. Serve the chilled pea soup in
individual soup bowls with a generous dollop of plain yoghurt and
some crusty bread.