Thoroughly scrub cucumbers. Remove stems and blossoms; slice off blossom ends. Cut cucumbers lengthwise into quarters.
For a charcoal or gas grill, place cucumbers and onion slices on the rack of a covered grill directly over medium heat. Grill about 6 minutes or until grill marks appear, turning once halfway through grilling. Cool slightly; separate onion slices into rings. Pack cucumbers, onion rings, and garlic into three sterilized pint canning jars or other jars.
In a medium stainless-steel, enamel, or nonstick heavy saucepan combine vinegar, the water, sugar, pickling salt, mustard seeds, celery seeds, turmeric, and crushed red pepper. Bring to boiling over medium-high heat, stirring to dissolve sugar and salt. Remove from heat.
Pour hot vinegar mixture over cucumbers and onion, making sure to cover vegetables. Cool for 30 minutes. Seal and label jars. Chill for 1 to 2 days before serving. Store in the refrigerator for up to 1 month.