Canning method
Wash the cucumbers. Cut a 1/16-inch slice from each blossom end and discard it. Cut the cucumbers into 1/4-inch slices.
Combine the vinegar, sugar, salt, celery seed and mustard seed in a large saucepan. Bring the mixture to a boil.
Place 2 onion slices and 1/2 dill head in the bottom of each clean pint jar. Fill with cucumber slices, leaving 1/2-inch headspace. Add 1 onion slice and 1/2 dill head on top.
Pour the hot pickling solution over the cucumbers, leaving 1/4-inch headspace. Apply the lids.
Process pint jars in a boiling-water canner for 15 minutes at 0–1,000 feet, 20 minutes at 1,001–6,000 feet, or 25 minutes above 6,000 feet.