In a large bowl, combine the sliced cucumbers and onion. Sprinkle 1 teaspoon of salt over the vegetables. Toss gently and let them sit for about 10 minutes to draw out excess moisture.
In a saucepan over medium heat, combine the sugar, apple cider vinegar, mustard seeds, celery seeds, turmeric, and black peppercorns (if using). Stir until the sugar has dissolved.
Bring the mixture to a gentle boil, then remove from heat.
Drain the cucumbers and onions, rinsing them under cold water to remove excess salt. Drain well.
Pack the cucumber and onion mixture into clean jars, filling them tightly but leaving some space at the top.
Pour the hot brine over the cucumbers and onions, making sure they are completely submerged. Seal the jars tightly with lids.
Allow the pickles to cool to room temperature, then refrigerate them. They will be ready to eat in about 24 hours but taste even better after a few days.
For an added kick, consider including a pinch of red pepper flakes in the brine.
The pickles can be stored in the refrigerator for up to 2 months.