Add the oil, onion and garlic to a large pot and sauté for 10-15 minutes.
Add the parsnip, carrot, celery and Brussel sprouts. Sauté for at least 5 minutes or until you see a faint orange colour released from the cooked carrots.
Next add the mushrooms, potato, parsley, reserved celery leaves, and half a green capsicum. Add the water, bouillon cube,frozen peas and tomato paste. Season with salt and pepper, and bring the soup to a boil. Once boiling, turn the heat down and simmer with the lid on for about 20 minutes.