Pea, mint and spring onion soup

Ingredients

• Olive oil – 1 tablespoon.
• Spring onion – 1/ 2 bunch (It should be sliced and you can keep a few extra to serve).
• Butter – a walnut-sized lump.
• Hot vegetable stock – 1 liter.
• Frozen petits pois – 900 grams.
• Potato – 1 no. (It should be cut into small size).
• Finely grated Parmesan – 85 grams. (You can also choose a vegetarian alternative).
• Mint, leaves picked – 1/2 small bunch. (You can also keep a few extra to serve).