Peel all but one of the cucumbers (the peels can be tough, so you only want a bit of peel in there to provide flecks of green colour). Cut all of the cucumbers in half lengthwise, then use a teaspoon to scoop out the seeds. Cut the resulting cucumber boats in half again lengthwise, then crosswise into ½-inch (1 cm) slices. You will need 16 cups of slices.
Pulse the cucumber slices in a food processor until they are chopped to fine chunks (but not too fine, or they will become mush); however you prefer them for relish. You will need to do it in batches. Dump the ground cucumbers into a large plastic, enamel, or glass bowl (salt can corrode some stainless steel if left in it overnight). Add the pickling salt, stirring it in to the cucumbers.
Cut the onions into chunks and process them to similar sized small chunks. Stir them into the cucumbers.
Quarter and core the apples (leave the peel on) and process them to similar sized chunks. Stir them into the cucumbers right away, making sure they are well combined and stirred into the salty juices so they don't get brown.
Let the salted mixture sit out on the counter for at least 8 hours or overnight.
In the morning, pour the mixture into a large colander and let it strain for 30 minutes. Don't press on it. Dump the strained mixture into a large heavy-bottomed stock pot (discard the liquid strained out).
Add the sugar, turmeric, ginger, white pepper, mustard seeds, celery seeds, vinegar, and 1 cup (240ml) of the apple juice.
Stir together the remaining ½ cup (120ml) of apple juice and the cornstarch in a small bowl and set aside.
Bring the relish mixture to a boil, then reduce the heat to medium and cook it for 15 minutes, uncovered.
Stir up the cornstarch slurry (which will have settled) and pour it into the relish, stirring to disperse it evenly. Bring the relish back to a boil.
Ladle the hot relish into hot sterilized pint (500ml) or half-pint (250ml) canning jars, filling them to within ½ inch (1cm) of the top. Run a butter knife down the sides to release any trapped air bubbles. Wipe the rims of the jars with a clean damp cloth, top with the hot, sterilized snap lids, screw on the lid rings so they are finger tight (only as tight as you can get them using just your thumb and index finger).
Process in a boiling water bath for 10 minutes.
Allow to cool, and store in a cool dark place. Leave the relish to age for at least one week before using it, to allow the flavours to mature.
Makes 6 pints plus a little left over to keep in the fridge for eating now.