Place 3 cups cucumber in a medium glass bowl; top with 1 cup onion. Repeat procedure with the remaining 3 cups cucumber and remaining 1 cup onion.
Combine vinegar and remaining ingredients in a small saucepan; stir well. Bring to a boil; cook 1 minute. Pour over cucumber mixture; let cool. Cover and chill at least 4 days.
Note: Pickles may be stored in the refrigerator for up to one month.