Pistou soup

INGREDIENTS

• 2 tbsp olive oil
• 1 onion , finely chopped
• 2 each medium carrots and celery sticks, peeled and finely chopped
• 1 small fennel bulb , trimmed and finely chopped
• 250g each turnips , celeriac and parsnip, peeled and finely chopped
• 140g frozen peas
• 140g tinned haricot beans or dried haricot beans soaked overnight
• large bunch basil
• 1 clove garlic
• 100ml extra-virgin olive oil
• grated Parmesan and croûtons, to serve