Layer cucumbers, onion, and salt in a large glass or ceramic container; cover with ice. Let stand 2 hours. Drain and rinse cucumbers and onion 3 times in cold water.
Combine remaining ingredients in a stainless steel stockpot. Bring to a boil; add cucumbers and onion. Reduce heat; simmer, stirring occasionally, 30 minutes or until cucumbers are translucent.
Pack pickles into hot sterilized jars; cover with syrup, leaving 1/2-inch head space. Remove air bubbles. Cover at once with metal lids, and screw bands tight. Process in boiling-water bath 10 minutes.