Break cauliflower into small flowerettes and place in a pot of rapidly boiling water for no more than 5 minutes. Immediately drain and plunge blanched flowerettes in chilled water. Leave to cool while you prepare dressing.
Peel and finely slice onion. Finely chop celery and thinly slice the olives.
Prepare dressing in a bowl large enough to accomodate cauliflower. Combine oil, lemon juice, capers, olives, onion, celery, parsley and salt and pepper to taste. Mix well. Add drained cauliflower, toss well.