Finely chop the onion, garlic, carrots, celery, pepper and chilli (if used).
Heat about 1 Tbsp of oil in a saucepan and gently cook the chopped vegetables until the onion is transparent and the carrot & pepper are tender.This should take about 8-10 minutes. Add the curry powder, stir for 30 seconds or so,
While the vegetables are sautéed, peel the celeriac and cut it into small cubes. Add these to the curried vegetables, stir through, then then pour in the stock. Bring to the boil, then cook under cover for about 20 minutes until the vegetables are tender.
Turn off the heat, add the mint and lemon juice.
Blend the soup until you get a silky, smooth texture.