Trim and discard the blossom ends of the cucumbers, then peel the onions and cut both into rounds about 1/4 inch thick. In a nonreactive bowl, toss them with the salt, then cover and refrigerate for 1 to 2 hours. Drain well, rinse, drain again and then set the cucumbers and onions aside.
In a nonreactive pot, combine all the remaining ingredients and bring to a boil over high heat, stirring once or twice to dissolve the brown sugar. Reduce the heat to low, simmer for 3 minutes and then pour the liquid over the cucumbers and onions. The cucumbers should be amply covered or slightly afloat.
Allow to cool to room temperature, then cover and refrigerate. These pickles have good flavor as soon as they are cool, but the flavor will deepen if you let them sit for 24 hours. They will keep, covered and refrigerated, for a month or more.