Toss together the cucumber slices, red onion, and salt. Refrigerate this mixture overnight. Rinse well, then squeeze dry in a clean cloth or press well against a colander to remove as much moisture as possible.
In a glass, plastic or ceramic bowl, stir together vinegar, sugar, and spices. Add the cucumber and stir well with a wooden spoon or your hands.
Pack into glass jars, leaving 1/2-inch of headspace, and freeze at least 24 hours. Thaw pickles in serving bowl for at least 10 minutes just before serving.