Melt the butter in a saucepan and gently fry the onion, carrot and celery until soft. Do not allow to brown. Stir in the flour and curry powder. Cook for 2 minutes and pour in the stock.
Bring to the boil, stirring constantly. Reduce the heat, cover and simmer gently for 30 minutes, stirring occasionally.
Peel, core and dice the apple, then add to the soup with the lemon juice and rice. Season to taste and simmer for a further 10 minutes. Serve hot garnished with a sprinkling of parsley.