Wash the cauliflowers and break them into flowerettes.
Blanch the pearl onions in boiling water for about 2 minutes. Cool and peel the onions. Mix the vegetables with the salt, add just enough water to cover and let stand about 18 hours.
Drain the cauliflower and onions, rinse in cold water, and drain again.
Dissolve the sugar in the vinegar, add the mustard seeds, celery seeds and hot pepper and bring to a boil. Add the vegetables and simmer for 10 minutes, or until the vegetables are barely tender.
Pack the vegetables into hot jars, fill the jars with the boiling-hot liquid and seal. Makes 4 pints.