To make the sauce: Drizzle the bread with the vinegar and enough water to just cover. Soak briefly then squeeze out and discard the liquid.
Put the soaked bread in a blender with the garlic, parsley, celery leaves and half the oil. Mix well then add the remaining oil and mix again. Season to taste with salt and pepper, and then thin with a little water until you have the consistency of a pouring sauce.
The cauliflower: Cook the florets in lightly salted boiling water until just tender. Drain well and place into a warmed bowl. Pour the sauce over, stir gently and serve.