Country vegetable soup with basil pistou

Name: Country vegetable soup with basil pistou
Ingredients: 2 stalks celery; 2 carrots; 4 tbsp/2½fl oz good-quality olive oil; 1 large onion; 2 small potatoes (such as Maris Piper); 4 garlic cloves; 1 ready-made bouquet garni (such as thyme, parsley, rosemary and sage); 2 litres/3 pints 10fl oz water; salt and freshly ground black pepper; 200g/7oz canned haricot beans, rinsed and drained; large bunch fresh basil leaves; 100g/3½oz parmesan; 100ml/3½fl oz good-quality olive oil, plus extra for drizzling